Most PEOPLE can't stay faithful.
I think a lot of people marry for the wrong reasons...to conform to society's expectations. My friend's colleague is about to pop the question to his girl. Her parents are pressuring her to get married and she makes more money than the guy - a lot more - so she looks great on paper and his parents are pressuring him to marry her as well. He started dating her because she really liked him, but initially, he was into her best friend! She was his back-up and he isn't all that into her, but again, she's good on paper. Perfection doesn't exist, but unless you're really certain that you can spend your life with a person, and you WANT to marry that person without outside pressure, then I don't think marriage should be in the books. For a lot of people, marriage is just the "next step" but that's not how it should work. You have to absolutely want that commitment and take the vows seriously. If you are a sexual person and your partner isn't, then don't marry him/her because obviously, it's not something that is going to change and at some point, you're going to get sick of putting your needs aside and you may stray. You may not because you take the vows seriously, but a lot of people act out of need/attention rather than staying true to their core values.