Because Mitchell is a silly boy. Contains very vague general references to plot points but no specific spoilers, unless you count information about his backstory that unfolds as the series does.
Mitchell vs Humanity:
Mitchell kind of has a cycle where he wants to be normal and fit in and thinks humanity is awesome and he should be as close to human as he can and then something happens which swings him right back to "humans suck and are prey" or at least "I can't control myself so I might as well not try". This has more or less been going on since the 60s, possibly before that. It's very, very easy for him to slip into becoming a predator, and that's because of two main factors. There's his dark passenger Noah inner vampire whispering at him basically all the time, but less obviously there's also the sheer social influence of having been with the vampire community (and Herrick in particular) for most of the last 90 years. Herrick treats Mitchell's departure from the group as an eccentricity- it's something he does every now and then but he always comes back to the fold eventually. So he's been surrounded by people who are dismissive of humanity at best and hateful at worst. This means that even when his addiction is relatively quiet and his opinion of humans is generally good, he's still effectively a casual racist when it comes to humanity.
When he's at his best, he very rarely slips. It mostly manifests itself in the fact that he has a tendency to view humanity as an undivided whole, and condemn them all for the actions of a few. He still uses pro-vampire words like "recruit" for turning someone (I think everyone on the show uses it but the only non-vampire who does that I recall is George, and he says it in a way that implies he picked it up from Mitchell), which is never questioned on the show but in camp he's kind of embarrassed when people point out his use of the word. As he swings towards disillusionment with humanity he starts hanging out more with his vampire buddies and spending less time at the house, which means he's more surrounded by anti-human sentiment and starts to spout some of it himself. At his most anti-human we see first-hand why Herrick recruited him in the first place ("I could see in you a great man. A terrible man. An orphan-maker, a breaker of hearts").
There's an interaction between his vampirism and the influence of vampire culture, naturally. It takes a lot of willpower to suppress his addiction, and the longer a vampire does it the harder it gets. The show only shows it being successful if a vampire is in love with a human (although I'm not sure this is the only way. Mitchell thinks it is but he's kinda selfish like that) and even that only works for a couple of decades. But anyway, Mitchell's actually pretty good at suppressing that side of himself when he wants to, especially when he's clean. If he's recently drunk blood (and depending on the amount, "recently" can be three weeks later), then it's much more difficult. It mostly manifests itself as rage and a desire for violence- there's a scene where you can practically see him mentally talk himself down from wanting to tear someone's throat out. The visual cue for him giving in to vampiric rage is his irises turning black. Vampires can also turn their entire eye including the whites black, but that can be done at will and seems to be an intimidation tactic whereas the iris thing is involuntary.
In camp, he's currently swinging towards the humans suck, vampires are awesome end of the spectrum. Especially since George is gone, he's mostly talking to other vampires and not humans- and even most of the humans aren't exactly normal humans so it's easy to think of them as not counting. Really the only human he talks to with any kind of regularity is Rictor, and it's easy to make an exception for one person (not that Rictor's view of the world helps his views on humanity in any case) He doesn't hate humanity or anything, but he's a bit more likely to come out with something disparaging towards them than he would otherwise, especially when talking to other vampires. Part of this is the influence of his talks with Maladict. Maladict sees vampires as a minority culture who are able to discard the whole killing people thing without losing their culture or assimilating into humanity. Mitchell hasn't got his head around this idea. To him vampire culture is inextricably linked with killing- he did try to change this but the attempt ended in disaster and he still hasn't worked out his feelings on whether it's actually possible or not given what happened. His holy grail has always been to be human (I mean it's only the title of the show), and Maladict's brand of proud inhumanity is strange and weird and new to him. He hasn't got his head around the idea that vampires can be awesome and humans can also be awesome at the same time, it doesn't need to be one or the other. So for the moment when Maladict gets him to think of vampire culture as something to be proud of, that translates into superiority, which as we know leads to all kinds of badness.
Mitchell vs Sexuality
I mentioned a while back in a previous essay that sex and drinking blood are very linked for Being Human vampires (LIKE PRACTICALLY EVERY OTHER VAMPIRE MYTHOLOGY EVER), and that Mitchell's very character is based on the concept of a recovering sex addict. So this one's going to be more about sexual orientation and how Mitchell's perceptions of his own are affected by his vampirism.
In a nutshell, I play Mitchell as bisexual but thinking of himself as straight with bisexuality as a side effect of being a vampire. He was born in 1894, and as far as I can tell sexuality wasn't categorised at that point (wiki tells me that Kinsey was born in the same year) and being anything other than "going to marry a girl someday" was pretty bloody weird at best and sinful and illegal at worst. He was a dorky sensitive type as a kid and canon tells me he's spent his whole life since trying to fit in with the cool kids, so my headcanon is that he totally bought into the whole enlist in the army to become a real man!! propaganda of WWI and that's why he joined. He was invested in being properly manly and the idea of doing anything with another guy would most definitely not have been part of that. And then he met Herrick in a forest in France, and was turned into a vampire (on a related note, my headcanon says he was a virgin when he died because he was too naive to want to have sex before he was married and too dorky for any girl to look at him twice in any case. I assume canon would have told me by now if he was married).
We actually outright hear Herrick's attitude towards vampire sexuality in canon: "A vampire is the only truly free man. All his darkness, all his excesses, they can run amok. He wants a girl, take twenty! He wants a boy, go ahead, just give the place a bit of a hose-down after." This is obviously about killing, but they're pretty similar things in this case. So as far as Herrick is concerned, vampires get to have sex with men because it's part of their whole hedonistic freedom that humans are forbidden. Mitchell was his right hand man and best friend for most of around 90 years, so there's no way he wasn't influenced by that opinion. We're shown that vampires often if not usually have sex with their victims and also with each other while they're at it, and the way vampires talk about their exploits reflects this, so it's fairly safe to say he's had a lot of sex with all kinds of people, but doesn't see that as part of his own identity. Vampire orgies are just the done thing! He probably had sex with Herrick plenty of times, but certainly doesn't consider him a lover. Emotions have nothing to do with it, it's just part of what one does.
Emotions only really come into play when it comes to trying to quit blood. The first time he seriously manages to stop for a while is also the first time he dates a human woman, because she's willing to help him stop. He seems to have taken away from this that in order to stop drinking blood successfully he needs someone (ie a woman) to change for, and to use to help himself stop. Someone actually asked about this at the san diego comic con panel and I love them for it. The panel's response was that they don't think Mitchell uses women specifically, he uses pretty much everyone close to him (Toby Whithouse (the creator of the show) added that he doesn't think it's necessarily a bad thing to use the people close to you like that but I don't think he got the point of the question really). But I agree more with the person who asked the question that the show explicitly shows him relying on the women he's in love with to stop him, much moreso than he does Annie and George (he's never outright asked them for help with it, much less begged them to save him). Anyway, he associates relationships with getting him away from blood, and it's never occurred to him that he could love a man outside of the whole vampire business.
There is one exception to the emotions-are-for-women-only business, and I think Mitchell sees him as an exception too. In season two they introduce a vampire called Carl who Mitchell lived with sometime before the 1990s, who also believes in abstaining from blood. In conversation with George, Mitchell says he lived with Carl for a while one of the times he tried to stop drinking, then after Mitchell moved on Carl lived with a human man, and Carl and the human were lovers. As far as I'm concerned the subtext in the scene is pretty clear that Mitchell and Carl were lovers while they were living together, and when Carl turns up in the show Mitchell treats him with a similar level of affection to another character who he canonically dated in the 60s who turns up decades later. But I don't think he sees Carl as "counting" in terms of Mitchell's conception of himself as straight, because Carl is still a vampire and they were still being vampires together, even if there was real emotion mixed in. At one point when Mitchell is in the throes of withdrawal he calls Carl's human lover a queer in a way clearly meant to be an insult, which I think shows that he really doesn't identify with the label himself (I don't think it makes him homophobic though, not under the specific circumstances of the scene. If anything it was meant as an anti-human sentiment).
So, Mitchell thinks of himself as straight because he associates himself having sex with men as a vampire thing and not an emotional thing or a thing he wants to do because he just wants to. He has had a relationship with a man before but that was in the context of two vampires trying to stop drinking blood, and he doesn't think it counts. He sees women as potential romantic partners and leans towards them as sexual partners even when he's being a predator, but if he hadn't been brought up with straightness as his only option and then had that mostly confirmed for most of his time as a vampire, he'd probably see himself as female-leaning bisexual.
I've gotta say though, this is one case where I'm really hoping to be jossed by him having a male love interest next season. Actual bisexual visibility, it's still possible!