Mishima needs to learn to value and love herself first, otherwise she will always be making bad decisions in pursuit of validation from men. I also really don't think it's fair to refer to Mishima in demeaning ways such as saying shes a thot or a homewrecker, at this point all of them except Kaeda can be categorized as homewreckers. I know it's easier to categorize her as such since this isn't her first time, still, I find her pitiful and this might also be a result of a poor upbringing (although can't know for sure, might be reaching). Also, I think the heavy drinking culture in japan isn't really helping out the whole affair problem.
You definitely isn't going to sleep with her this time, firstly, she is heavily intoxicated and clearly at a mental low point. Secondly, he is more afraid of losing Michi than ever right now, and thats more than enough to keep him from doing anything stupid (for now).