Amanda, I'm so sorry you have had this huge shock and disappointment. Some couples do patch up after such things, I don't know if I would. It's the deceit more than anything that would get to me, I would never feel sure I could trust him again. I think you have to trust your gut instincts about things like this, at first, anyway. You most definitely need time to think about it.
How he responds now makes all the difference too. Not just being sorry, swearing it will never happen again, etc, but real conversations that go deeper than before, a real search for the reasons he has behaved this way and his truly desiring to change his behaviour. Simply apologising and fluffing isn't enough.
Whatever happens and whatever you decide to do make sure that you don't settle for being treated disrespectfully.
Support is here on mumsnet in spades, as I'm sure you know xxx