How do you do your part to fix a serious problem in a relationship that you didn't cause?

Example: I've been with my husband 8 years. We found out were expecting and I am now 21 weeks along. I recently found out he has a problem with porn and talking to some of these women via internet. I'm at a loss for what to do. Do I stay and trust he will do what he says and change? Do I make him stay at his mom's a few days? Do I leave for good? I can't fathom giving up on our marriage and soon to be family but I don't want to waste time only to be made more of a fool.