Because that's the way they are wired.
Your "theories" are ludicrous for several reasons:
1. They are all based on the assumption that, if a lady won't do what her guy wants, the guy will easily turn to another man to achieve satisfaction. Why not stuff a cucumber up his butt or blow a goat if that's all it takes?
2. They assume that having sex with guys is the same as having sex with women. It is not...the experiences are quite different.
3. They suppose that there must be a "reason" for a man to be bi sexual other than the fact that he quite simply IS.