Many cases of infidelity? I find that hard to believe.
I would like to point out that just because the marriage seems fine to you doesn't mean it is to your spouse. He/She may beg to differ with you that it wasn't crap. Marriages are tough, they aren't always fun, and sometimes they can be downright crappy. Try to honestly remember your marriage and throw the rose colored glasses in the trash.
You cannot be lonely if you like the person you're alone with. Dr. Wayne Dyer