1 - Desiring someone else ? That's pretty normal. Nothing wrong here.
Now, acting on it ? Cheating is one of the most contemptible, mean, pathetic and evil thing to do to someone.
I consider every person doing it willingly (save for the few weird situations where there is an actual reasonable reason to do it) to be human garbage.
Of course, I'm talking about cheating. If someone is in an "open" relationship, there is no "cheat" involved.
2 - Both are scumbags, but I consider the person in relationship to be quite a bit more responsible. (s)He's the one with an involvement and accountability toward someone that has invested her/his faith in her/him. It's certainly more heinous to betray someone who trust you and which with you have a relationship, than to trample over someone you have no relationship with (doesn't mean that the latter is trivial though).
3 - This is completely irrelevant.