Younger guys: less than half, possibly even less than a quarter. A lot of my younger clients are seeing an escort because they have a hard time getting into a regular relationship but still crave sex and companionship. I end up giving these clients a lot of dating advice. (I'm kind of tired of giving dating advice; there's only so many times you can say "Join an activity that will force you to talk to people" or "It's better to ask someone out right away than hope she magically realizes you like her" before you start to grit your teeth.)
Older guys: At least three-quarters. I dislike that they're cheating — breaking your promises to your partner is a shitty, shitty thing — but, although it doesn't excuse them, nonmonogamy is only just starting to edge into cultural acceptability. The option for "have an open, honest, nonmonogamous relationship" just wasn't there when they were getting married, and I imagine it's hard to break off a loving, long-term relationship because of something that's easy to cover up.