Wow - that's really judgmental and messed up. First of all, it's his wife - not his girlfriend and they have young children to think about. So it's not at ALL on the same level of being a playa when you're young and single.
The wife is clearly selfish and shows no regard in the least for her marraige, children and family. And in this case she couldn't more clearly be the sexua| predator - and with tragic consequences.
Most of us know that this marriage (like so many others) is doomed - but with kids at stake, the OP will never feel good about himself if he gives up without a fight either. But this will not be a happy marriage or happy life moving forward and at some point he's got to swallow the pain, grieve and move on. Lifelong unhappiness is never something you should knowingly settle for.