Thishasreallyupsetme
I'm posting this as TashaE's husband.
A bloke's perspective on all this...we don't always have motives for everything we do...we just think differently sometimes.
I know you're in a really difficult position, but you need to take a step back from doing something you will really regret later. In between all the messages about how hurt and upset you are, you have said that you love DH and that he is a good father and from the sound of it, you don't really want to throw all this away.
You (and almost everyone else on MN) are saying your DH is behaving really badly (and to an extent, I agree), BUT, there are two sides to this. First, you have mentioned above the odd relationship with MIL, so this clearly isn't the first time and it isn't something that is going to change; you cannot simply 'divorce' someone from their parents, however odd they are, so you will simply have to live with this. Secondly, from your DH's perspective, he has been put in a difficult position by his mother and he probably simply didn't know how to broach the subject with you because he knew it would hurt you - blokes don't find this sort of thing very easy to talk about and the easiest thing to do (and in hindsight, the wrong thing - isn't hindsight wonderful!) is not to tell you and hope the problem will just go away.
I am sorry to say this, but you have now boxed him into a corner...you need to give him a way out of this that allows him to say sorry to you in his own way. So, don't confront him any further tonight.
Tomorrow morning, start a conversation with "DH, I know you've been put in a difficult position and I know you were going to tell me...I was just a bit shocked and hurt to find out about this the way I did. Can you tell me what's happened..." and take things from there.
Finally, I have no idea what your tone was when you said this, but "we need to talk about this" can mean exactly that, or it can mean "YOU'RE GUILTY! JUSTIFY YOURSELF". If you mean the former (and I think that, in your heart, you do), you need to make that clear.
Good luck.
Husband, who will never be allowed on MN again!