His story is really implausible.
Sorry but I agree with clearguidance. There are red flags all over the place. It is really hard to see when you haven't been through it, but this is a typical response. After almost 4 years here I've seen alot of BS go through this.
At first they deny, deny, deny.Then its we were just friends.Then it was just a kiss.Then, we had sex but only once.And finally the full blown affair comes out.
I'm sorry that you are here and I am afraid the worst is yet to come. Looking back I never would have believed that my husband was cheating but in retrospect now it was so clear. We too had three small kids, actually 2 and a half as I was pregnant at the time. He was 35 and she was 21.
You need to do some investigating as I'm sure there is more there. Check the cell phone and text records. Investigate the history on the computer. Put a key logger on. Get one friend you trust to follow him.
Spend a long time here reading. You are not alone. This is a great group of people who have been through hell and will give you support. You will learn so much about how WS's act, the lies they tell and how they manipulate to make you think you are the crazy one. Been there, done that, wear the t-shirt.
Good luck and come back when you know more. We are here.
I completely agree with the previous posters.
I'm right with the other posters.