cheating doesn't just sort of "happen," unless your penis magically appeared in her mouth one night and you were both shocked and horrified. I think you're right, that people don't go out looking to cheat, but it's a conscious decision that both parties make.
marriage, or any relationship really, takes work, but using that to excuse running off on your wife and kids is low. you said yourself that breaking up a marriage is incredibly hard, and yet did it/came very very close for "some chick from class" (your words). your wife has every right to not trust you, since you clearly strayed without much reservation... I think you understand what you did was wrong, but seeing you justify it like this is a little gross. if I'm wrong about any of this, feel free to correct me.