Okay, here’s the thing.
Yes, I ship Snape/Lily, in large part because I feel sorry for Snape in the end and think he could have used a little happiness in life. But, to be honest, I think Lily would have been better off alone.
Snape’s an ass, sure. They were friends as kids, but Lily was right to turn him away when he apologized, because she was right about his motivation. Perhaps if she’d given him time to pull his thoughts together he might have changed, but in a very real way she was protecting herself by walking away from him.
That said, James Potter’s a worse ass (and Sirius is worse still, but that’s another rant). He’s a blatant bully that should have been expelled for what he was doing to Snape in school. Yes, Snape retaliated, but James and his friends ganged up three or four to one and, among other things, stripped him in public. James did save Snape from Lupin, but not doing so would have been tantamount to condoning murder--so really, no bonus points for acting like an actual human.
James was nice to Lily. But would you want to live with someone who treated anyone the way he did Snape? It’s not just the moral issue; how long before that cruelty would be turned on his partner?
Snape’s no fit lover, but I submit James isn’t either. Lily should have left them both behind, and found a better person if she wanted one.