I don't usually do fandom rants like this, but I feel like I should put this out there. Okay, so. It's not often I see Bruce being played, or even written about, though perhaps that's because I don't look all that hard. But when I do, there is something I see that often bugs me. Bruce Banner is not best friends with Tony Stark. They're not gay for each other, they're not going to move in and have breakfast in bed on Sunday mornings.
Yes, they are going to be on the Avengers together, as we've already seen from the bonus scene at the end of The Incredible Hulk that Tony has been assigned to track down Bruce. Now, assuming that Tony does track Bruce down in Bella Coola, British Columbia and corners him long enough to offer him the spot on the team, how would Bruce react? Considering that Bruce has been running for at least six years(*) in order to prevent Ross from capitalizing on the Hulk for military use, would he really, really just change his mind because Tony Stark is asking him to? Remember that Bruce would have no knowledge of Tony's change of heart (ha, irony), his experience in Afghanistan, or of Iron Man. He would only remember Tony based on news from circa 2002 - back when he was still the womanizing, egocentric, party-hopper arms manufacturer. It's a very real possibility that Bruce would see little to no difference between Ross and Stark, both military-aligned men angling to get their hands on the newest weapon.
Even if Tony whipped out the kind of impassioned, heroic speech like he gave to Pepper after she found him in the suit, it's doubtful Bruce would believe it, for several reasons. Firstly, his prior knowledge of Tony Stark's exploits - it would be as if Paris Hilton suddenly turned around and decided she was going to be the new Mother Theresa, and just as unbelievable. Secondly, Bruce would most likely not believe that this just wasn't another of Ross' ploys, or that Stark wasn't working with Ross in some way. Thirdly, and perhaps most importantly, Bruce doesn't see himself as a hero. He doesn't see the Hulk as a kind of power, or a gift. At best, he sees it as the worst in himself, all his anger and rage and id come to uncontrollable life - a kind of Dr. Jekyll/Mr. Hyde type deal. At worst, he sees it as something completely alien from himself, a kind of emotionally-triggered tumor that needs to be excised as soon as possible before it hurts more people. But either way that Bruce might see the Hulk is sort of a moot point as he cannot control himself while in the Hulk state(**).
What this all amounts to is that Bruce would not, given what we've been shown of him in the film, simply go belly-up to Tony Stark, or even Nick Fury. If anything, we've seen Bruce get a ridiculous amount of steel in him when it comes down to the wire - just watch his conversation with Betty in the truck after leaving the motel, or when he discovered Sterns' synthesized blood bank. So he would ultimately be saying no to any offer he was given.
...Unless. Unless he wasn't offered the position, but instead given an impossible choice. Now, Bruce's sole hope, the one thing that keeps him going through his self-imposed exile, is the hope that someday, somehow, he'll be able to cure himself and go home, to Betty and the life he used to have. If he was forced to see all the damage the Hulk has caused, the list of known fatalities, and basically have the fact that running around the whole of the western hemisphere with no steady access to decent scientific equipment wasn't working shoved in his face? Bruce could possibly be made to feel guilty enough to concede. This approach would be made even more probable if Bruce was offered the whole of Stark Laboratories to work in. Then again, that might not be offered as Bruce would be working towards curing himself of the Hulk and the Avengers would have little use or need for a Hulk-less Banner.
Another argument that could be made to force him is that two creatures have already been created (Hulk and Abomination) so it wouldn't be completely impossible to continue the process until it was perfected. Now, Bruce is already living with his monster, and he was angered/appalled at the knowledge that Ross had commissioned another creature which ended up running amok in Harlem. Putting it to him that his refusal to cooperate with Stark, Fury, et al will be consigning an unknown number of people to be de facto guinea pigs? Bruce Banner would not allow that to be on his conscience - he wouldn't allow other people to suffer because of him.
So however Stark and/or Fury manage to get Bruce on the team, Bruce would not be there entirely of his own free will. Call it what you like, but at best it's heavy-handed leaning and at absolute worst it's blackmail. Either way, Bruce would certainly not be frolicking around the happy hunting ground at being the newest member of the spandex squad. In probable fact he would be quite angry at being forced into such a position, with blame landing squarely on Tony's shoulders. Why? Because Fury mainly stays behind the scenes, with Tony as the more prominent head of operations - as seen when as opposed to having S.H.I.E.L.D. personnel looking for Ross after the Harlem incident, he sent Tony instead, or waiting until the middle of the night before breaking into Tony's home to speak with him.
Now, all those professional reasons aside, there are a number of personal reasons why Bruce would dislike Tony. One such reason is mostly based on meta-ly applied comic canon which hasn't been confirmed as standing true for the film canon, so I will not expound on that herein (though if you're curious, dear reader, I'm more than happy to expound in comments). The other reason, however, can easily be inferred just from comparing the two men and their confirmed history. To sum it up in a word? It's jealousy. Let me explain.
On the one hand, you have Bruce Banner. A brilliant scientist, rather vanilla, nothing all that special. He chooses to work on a well-meaning project meant to protect those who have to be sent into harm's way, to bolster resistance to gamma radiation for soldiers. A rather noble aim. He volunteers himself in order to move the project along more quickly, and is bitten in the butt for his troubles. Instead of allowing this newly created thing within him to be twisted into a weapon, he chooses to give up everything he knows and loves, simply to do what he feels is right. He continues to live this life for over six years, training himself, and avoiding most meaningful human contact in order to keep people safe, all while having nothing to his name but the clothes on his back.
On the other hand, you have Tony Stark. A brilliant engineer who's had everything he's ever wanted from the minute he came bounding into the world. He breezes through school, then spends a few years thoroughly enjoying himself with alcohol and women all while being supported by his family's money. Then he waltzes back into his father's company, designs and sells smart weapons that end up killing thousands of people, and still very enjoying himself with alcohol and women. He spends so much time enjoying himself that he doesn't notice or care about his company's underhanded dealings and only chooses to pay attention after that negligence rears up and socks him square in the mouth. After his three-month ordeal, he comes back, announces a halt in production that most likely leads to a number of layoffs and losses for who knows how many people, and proceeds to rebuild yet another weapon - which once again, due to his short-sightedness, ends up in the wrong hands! He becomes involved in a large fight with an enormous amount of collateral damage to both public, private, and his own property. The repercussions of all this? He comes out as a superhero who still manages to enjoy himself pretty thoroughly with alcohol and women while having more money than he could ever know what do do with. Never mind the main things he's fighting against are his own designs and technology that ONCE AGAIN end up in the wrong hands!
Do you see where this comparison is going?
Bruce has done his best to be responsible, do the right thing, hurt as few people as possible, and what has it gotten him? Nothing but ordeal after ordeal after ordeal, and even now he can never, ever lose control or people will be hurt. He can never fully relax with the Hulk just waiting to burst through. Tony, on the other hand, has appeared to have done his best to be irresponsible the majority of his life, made a living off of hurting people, did what he wanted whenever he wanted with reckless abandon, and it's gotten him the best of everything. Even the most kindhearted, generous, optimistic person would find it hard to say anything when the two records are put side by side. Bruce most likely wonders just what it is that allows Tony to live his charmed life, what quality Tony has that he lacks to make this all possible, and is most likely rather ticked off that this highfalutin', freewheeling playboy gets all the breaks while he has to stay at home and meditate. It's just not fair.
Of course, Bruce doesn't complain about such things, and so would simply internalize this jealousy - though it certainly would bleed over, no matter how hard he tried.
Am I saying that Bruce Banner and Tony Stark will always be bitter enemies? Not at all. I think if Tony ever really opened up about what he went through in Afghanistan with Yinsen, the complete betrayal by Obadiah and how that truly hurt him, that Bruce would empathize with him, and start seeing the real Tony beneath all the flash and glitter. But the thing is that Tony doesn't open up about those things. I would highly, highly doubt that he's ever told Pepper about the full impact of those events, and she's so much closer to him than Bruce would probably ever be. It would take several months of working with each other to develop the kind of trust and mutual respect needed for anything remotely resembling friendship. But until then, and probably even then, Tony would still be a brazen, arrogant showoff trying to get Banner to toe the party line, and Bruce would still be an irritated, sometimes bitchy guy chafing under the proscriptions that have been forced upon him.
As for anything overtly sexual, it's simply impossible until further canon states otherwise. Why, besides the fact that Tony so obviously has deep feelings for Pepper which she most likely reciprocates? And besides the fact that Bruce still loves Betty and she him after six years of separation with no word from either party? Because as was made so painfully evident in the film, Bruce cannot engage in sexual intercourse without physically exciting himself to the point of a Hulk transformation and Bruce refuses to put people (most especially his partner) in danger just so he can get off. The man couldn't even have a proper make-out session without his pulse monitor going off. In fact, it's easy enough to assume that he can't even masturbate without going over that threshold (another matter he most likely envies Tony for - six years with no release and this guy has a legion of scantily clad themed dancers, yes, that's totally fair).
I've done my best to keep my personal head-canon and interpretation out of this, and I think I've proven my point well enough. Discussion is always welcome though!
*The Incredible Hulk was released in June 2008, though production notes tell us filming began in July 2007, and it's explicitly stated that Bruce has been on the run for five years prior to the film, then figure three-four months for events in-film, then six months between Stark being approached by Fury in the end of Iron Man and the events of Iron Man 2. Assume the film is set in the relative-present and allowing for enough time that all the other heroes needed for the Avengers have been recruited with Hulk left for last? We're talking Bruce has been on the run for six years at a bare minimum. It'll probably be closer to seven or eight before S.H.I.E.L.D. catches up with him. Thus putting the accident around 2002-03.
**Now, you might argue that the semi-final scene in the film denotes Bruce having gained some level of control over the Hulk. Here I must disagree, and most emphatically. If Bruce was indeed attempting to trigger himself into a controlled Hulk state, why would he be inside his rather small cabin while fully clothed? Bruce has gone through a number of transformations prior to this - he would know that any clothes he was wearing would be utterly ruined by the transformation, and that he would have little to no room to move after said transformation. In fact, while meditating at the beginning of the film, we see him in a large indoor area and in minimal clothing, and this is when there's a minimal chance of triggering himself! He knows what the transformation entails if only from what he's experienced after waking up from it, and the one thing Bruce Banner is not is stupid.