Eh.
1st case: I wouldn't call him a jerk there, especially not to Sara. Marten, maybe. But Marten also didn't have any sort of deep love for Sara, she was just a girl he thought was cute, and now he's learned she wasn't interested. Plus Steve effectively asked Marten if he minded in that strip. I'm sure had Marten objected, Steve would've kept his distance. (Marten confronted him about his intentions before Steve had even come to a decision either way--he was just beginning to consider whether it was something he wanted to do--but I'm assuming "would Marten mind" was the main part of why he would even be weighing the options in the first place.)
2nd case: I don't think that makes him a bad guy at all. First of all, for all we know it was mutual. Not like a woman wants a relationship full of bad sex either. Second of all, so... he went out with her and found out they were incompatible. That is exactly a reason people *should* break up. Sex is important in a relationship: satisfying sex strengthens a relationship, and unsatisfying sex weakens it (there's been studies!). It's the same as if they broke up because they realized when hanging out, their conversation was awkward and never went anywhere. Also, having been there, I wanna say that although someone unskilled at sex can get better with practice, if your styles just don't mesh like what Steve was describing... that can't be fixed, that's just two people not being right for each other and having different physical rhythms.
3rd case: True enough, although even if it's what he says he'd do, he's not yet done it. Also being Steve, he'd probably fall for her afterwards anyway. (He doesn't know that Angus likes Faye in that comic, does he? I think if Steve liked another girl he'd behave like Angus did.)