Hi A.,
I'm so sorry this has happened. I agree with what has been written so far and just wanted to encourage you in your decision. I have seen people come out the other side of this with their marriages becoming stronger and better. And I read somewhere that when couples stay together for their kids sake they end up being happily married in the end. In other words, it passes and newer more positive experiences overpower the old, bad ones. Hopefully that makes sense I've got a toddler having a fit here over his blocks falling apart!
I wanted to add that forgiveness isn't easy, but it's important. You don't want to carry that poison around with you. Try to focus on any single thing you can that is good about him as you go through this, like he's good to the kids or works hard or something. But if you don't let it go it will eat you alive. This I have learned firsthand and it sounds cliche but is true. We don't forgive for them, we forgive for ourselves. I don't know if you're religious, but it helps to pray the Our Father because you say forgive us as we forgive others. And boy it's hard to hold a grudge when we're saying forgive me like I forgive others. I have let go of many hurts by praying that prayer.
Your case is extreme and I'm not trying to diminish that, just offering another look at forgiveness and why it's important. Having been forgiven some pretty terrible things from early in my marriage, I can say forgiveness is a beautiful gift to yourself and your kids' dad. And ultimately your kids themselves, because a family cannot be replaced, it can be copied but there is no substitute for both parents together in a home with their kiddos.
Good luck to you!
D.