>>105381173I'd see it as cheating if either Star had initiated it, since she was the one actually in a relationship at the time, or if she and Marco kissed literally all the time without Tom's knowledge. Or if instead of kissing they'd literally fucked, but obviously that's not happening.
As it is, Star immediately was like "oh fuck, what did we just do," seemed to regret it, Marco told Tom what happened, and as of right now nothing's continued from there. And yeah, of course they are going to end up together, but I mean they haven't been carrying on a relationship behind anyone's back.
They didn't do a whole lot wrong. Tom also barely seems to give a fuck about Star when those two are actually together, and so it's hard to see him as some sort of tragic victim when Star tries to talk to him about stuff she cares about and he's just like "yeah cool whatever" and then whines and walks off when Star doesn't go along with him trying to fuck her.