The problem is what you say is that the success of failure of the marriage rests on good, pro marriage behavior by both parties. A partner could go into the marriage as you say, and then just really screw up at a later juncture.
Look at this scenario: a good guy can be very respectful and sincere with a woman who doesn't want to have sex right away. He is patient and suportive, and they build a good relationship, get married, and enjoy a good emotional and physical relationship after that.
A guy can swoop in and seduce the woman in a matter of a couple of hours. A floozie can swoop in and seduce the man in a matter of a few minutes. That's not to say the decision to cheat is excusable, but it does happen. What's left is what the two parties (not one or the other) decide to do.
Sometimes the cheated-upon spouse's hands are not clean, either. While not a cheater, they might have alienated their spouse in some way that helped them fall to temptation swifter.
My standard advice for dealing with a cheating galpal or spouse is to leave them. I have a very utility-based argument for that, too, if anyone wants to know.
I'd be interested in hearing it...