Me and this specific friend, who we'll call Carl, have known each other for a while. We actually met because he dated my best friend(f18), but he wanted to get physical and she didn't so they broke up. After they broke up he expressed interest in me, and my best friend told me "I'm cool with it if you and him ever do anything, just don't tell me about it because that's weird,". My best friend doesn't know about any of this because I'm trying to respect that boundary. I have no desire to be in a relationship with him, and my best friend knows that.

Me and Carl decided to hang out one night and go for a drive. we had hung out before and it was always platonic with some suggestive comments that never led to anything, so I didn't think anything would happen. We would hang out once in a blue moon, mostly texted to keep up. I haven't had instagram in a while so I wouldn't see if he was posting pictures with anyone, but I figured if he was dating someone I'd find out either from him or his cousin who I am friends with. Well, we ended up stargazing and listening to the radio, he asks if he can kiss me, I say yes because I'm touch starved and I trusted him. One thing leads to another and we're having sex in the back of his car. After that we listened to music and got McDonalds. It was very sudden, and frankly I didn't mind that it happened. In an explicit way it was like I was just having fun with an old friend.

The next day I explained to him that I was not interested in making this a regular thing, and he asked if we could do it one last time and then keep it to ourselves. I was sure he'd already have told someone as he is kind of a kiss and teller, but I appreciated the gesture, and frankly he was pretty good. The next night it happened again at his house. This was a month ago.

This morning I opened instagram and logged in to my old account to wipe off the dust and see if I could find some use out of it, when I see a post by an account of an old friend of mine who I don't really talk to anymore(f19). It's a picture of her, Carl, and a baby. The caption read "Happy 5 months to my sweet baby boy, and almost 6 months with the man of my dreams, the best father my child could hope for!"

I was stunned. I still am. I don't know what to do, should I do anything? I feel awful, and betrayed. It's the only time I've had sex outside of a committed relationship, which I've only had 2. I trusted him enough to try something new and spontaneous, and he was using me. I can't vent to my best friend of 12 years about it because it's her ex that she only has ONE boundary regarding, which is "if you sleep with him just don't tell me about it".

I feel cornered and I don't know if I should tell his girlfriend, or anyone.

edit:typos