To Have and To Hold

Last night I was suffering from a terrible bout of Cabin Fever and was about to go out to a local bar down the street alone when my good friend Emma called and agreed to meet for a drink. We met at a bar called the Mystic Celt near her work where we had a couple of drinks and gabbed about life. When she informed me that she was going to have to leave soon, I jokingly gestured to a group of men standing outside and said, "It's cool... I am going to hang out with them the rest of the night."
Little did I know, that group of men was actually a bachelor party and they had seen me motion towards them; which inspired a couple of them to come over and chat Emma and I up. One in particular seemed rather interested in me. He introduced himself as the Groom-to-be's brother and invited me to join the group. Although confused as to how bringing a girl along to a bachelor party isn't considered a buzz-kill, he seemed very sweet. He also reminded me of Edward Norton, which I liked. So, despite what my mother would surely consider a "kidnap-rape-murder" scenario; I tagged along.
All of the gentleman were very nice to me. I had shots and beer bought for me and was hanging out with them like one of the guys. By the third bar, I was feeling good. Good enough, in fact, to partake in some drunken making out with the Groom's brother.
After several minutes of grossing out everyone else at the bar, we decided to get another drink. However, upon our return to the table, the rest of the party had disappeared. They were no where to be found.
So, being the considerate lady I am- I took the Groom's brother home.
From here, I don't have to get into too much detail. But I will say: I got laid. Twice.
He left this morning after plenty of cuddling, kissing and number exchanging. He said he really liked me and wanted to see me again. He left and I felt good.

The dumbass made one mistake though- and that was to give me his last name.

Living in today's day and age, of course the first thing I did was FB him. Mainly because I wanted to show my cousin what he looked like as I told her about my evening. I click on his profile and my jaw drops.

"In a relationship with.... "

Shit. Maybe it's a joke? I click on her name.

Picture of her, him and a child on some sort of intertube, waving and wearing life vests.

Under her interests? Motherhood.
Fuck, fuck, fuck.

"Hi, I am Nicole and I just slept with someone's boyfriend/ baby's father(?). Twice."
(This is where you all stand up and say, "Hi Nicole!")
Honestly, this is why so many men get a bad reputation. Assholes like this guy. He seemed so very genuine- only to find out he played me so very well.

The next question is, how much do I get involved? Do I tell his girlfriend? I don't think I will...just the thought of doing that is so gross. Plus, she might knife me. Because a lot of the time girls who have been cheated on don't listen to logic such as: murder is bad for you.
So, what should I do? Tell her? Let him think he got away with it? Add him on FB and make him feel super awkward?

I guess I should look on the bright side- at least he wasn't the Groom.