... because I am just a multiperson person.
... because my partner pushes me into it (constructive dismissal).
... because of strategic reasons.
There may be all sorts of reasons. I do know a few men who had secret "flings" but still in their marriages and generally there were two reasons:
1. A midlife or later life crisis thing where there was opportunity to have a young one - falling for the ego seduction. They tend to feel stupid after doing so.
2. He cannot get the kind of sex he wants at home, and that always meant no oral sex in situations I was told.
They don't teach how to have good sex in school or anywhere else. Many couples can't really talk about it either - maybe afraid to be embarrassed. A prudish spouse or just unable to fulfill the other person is an ego and emotional blow to both of them.
That doesn't make any cheat of course. But there are many causes that communication and effort could avoid. And some marriages/relationships are probably doomed from the start for that reason too.
If I remember correctly, our OPer is young, so probably is looking at it more from a BF/GF or theoretical marriage viewpoint, rather than married, house, mortgage, kids, cars, bills and being entirely entangled with each other. Then "cheating" is a BIG DEAL because so much is at stake and it is a betrayal against the entire package and so much of one's self invested in it.
Thus you could get the sense of "after all I did and sacrificed for you, all we did and had, and you jumped into bed with your young secretary!?"
I don't know any situation personally where a cheating spouse was driven to it by their partner, though. The ones I've known who cheated were complete assholes. I think it's a very convenient excuse.
**Please! Leave me alone! Ya'll know that's true.
Thank you, Quazi!