My grandfather always said that you have to be very careful of a habitual cheater. If they don't care about screwing over their family (no pun intended), they certainly wouldn't care what they might do to you.

Like I said, a promise is not meant to be broken. Anything can, and does, happen, though. People change or grow apart. However, you can't expect to stay married AND have a string of affairs. That's a bit TOO selfish.