I cheated on my Bf... i think? HELP?

SORRY THIS IS LONG. My bf and I have been dating seriously for 6 1/2 years. I finally got on Anti-depressants (7months ago) which brought my sex drive back... I've never done anything close to cheating up until 6 months ago, when I started to flirt. My boyfriend knew we were flirting but didn't care because it was harmless. My bf's friend gives me a lot of attention while im drunk and hes drunk. Hes not attractive to me just i like the attention. Well last weekend he asked if i wanted to see his you know and I was like i guess why not? Its something that everyone always joked about. I said thats too big and he said you don't know yet unless you touch it which i said no im good. but he asked like 3 times and eventually i quickly touched it. it was just for a second but right after I quickly went back downstairs with my bf and felt so uncomfortable.

He even texted me in the morning asking if i wanted a "special" picture for just me in the morning. When i said i can't because that would upset my bf he said he didn't want to get me in trouble and said he could send it privately on instagram. I said idk and left it there.

We hangout with this guy almost every weekend. What do i do? how do i cut off the flirting and tell him i'm super uncomfortable now without making it weird??? I can't risk losing the only person i've ever considered marriage and kids with. I feel like a terrible person. he does everything for me and idk what to do now.