The problem with you Mr. Box is you have a clear case of circular logic. You believe one thing to be true and then look for every reason to justify why you are right without accepting that you could be completely wrong, like when you say "seems to be the simplest explanation, thus it's likely to be correct." That is not good or sensible. First of all, just because you don't like Messy doesn't mean the girls feel the same way. Obviously Nastya likes him and Cristy had no problems fucking him, so obviously he's not as much of an asshole as you want to believe. Second, how exactly was it obvious for Cristy to have a threesome when she was basically throwing herself at Phil all of last night? If Phil would have fucked her, she would have never ended up in the other bedroom. Maybe I'm right, maybe I'm wrong, but nobody here can say for sure, including yourself. And trying to say what their tasks are when you do not understand the language is also just kind of silly.
It is also clear from your first statements that you are not too familiar with women. Aside from the fact that she was in bed with and fooling around with Nastya WAY before Messy joined them, it is very common for women to have sex with guys because they feel hurt or rejected by other guys. The term is called "being a rebound". Unless you're young or haven't been around a lot of women, that's pretty much common knowledge.