L.,
All I can say in this situation is go with your gut feeling. I was in the same situation. I was married for almost eight years. We had two beautiful daughters, nice home, everything great. Then after I had my third daughter, he started "hanging out with the boys", needing some "man time", which later I found out was a 19 year-old female.
I'm not going to go into a lot of details, but to make a long story short...There were a lot of times that he would hang out and come home late and always have an excuse, but I sware to you I ALWAYS knew that he was lying to me. It would just be this "gut" feeling as they say, and it would linger in my mind for days. I should've questioned him more and took more of a stand with the situation and not just take his word for a lot of it. I think that I was just didn't want to know the truth and have to deal with it, so I would just go with whatever he said. I had alot going on with the house and the kids and just didn't want to deal with that, but it ended up becoming an even bigger problem. We divorced in 2004.
Take action now, don't let it go. Talk to him. Communication is the key. I learned that the hard way. Find out what it is that he feels that he needs and expects of you. If it's just time out with the boys...that's fine, but he is married and needs to respect that. Tell him what it is that you expect of him and what you need also. Let him know that this is a problem. There is absolutely no reason for a married man to be at the movies with another woman and i'm sure he didn't tell you in advance.
If he is out and comes home explaining to you where he has been and it just doesn't feel right to you...question it! Don't worry about him getting upset with you for asking questions because YOU are his wife and have the right.
One last thing...start to take a little more time for yourself. Just because you have the babies doesn't mean that mommy can't have a girls night out every once in a while. Get dressed up and go out. Have fun. My ex used to have a fit when I would go out, which was more like once a month, but he would seem to come and go as he pleased. Also talk to him about you two having some private time. You need that. If you take a stand now you've got a good chance of saving things because the road you're headed down is all to familiar to me.
Feel free to email me at ____@____.com if you ever want to talk. Hope i've been of some help.