Friends with benefits

So my ex and I broke up over 6 months ago. We are really close friends now and in the past few weeks we became “friends with benefits”. Meaning, we aren’t dating but we still do sexual things, but we can also do them with other people.

On the weekend I made out shirtless with a guy in his bed. My friends with benefits found out that I made out with someone and keeps asking who it was. The guy and I agreed not to tell anyone though.

My friends with benefits won’t stop asking who it was and he seems kind of mad or uncomfortable that I made out with someone else.

I didn’t even mean to tell him that I made out with someone but it slipped up.

Why would he be mad? He does things with girls online all the time.