Why do people get married when they know they can't be faithful?

People don't just wake up one morning and say, "you know what, I feel like voiding my vows and cheating on my spouse". Why even waste your time and your souse's time by getting into a marriage that's a known sham? Correct me if I'm wrong but I don't think the are any current laws in the U.S. that force people to get married, so why get married and be half a**ed about it? There's nothing wrong with living the bachelor/ette lifestyle if you don't want to commit. But why would you agree to marry someone without the intention of trying to remain faithful?