I just found out that my friends with benefits has a girlfriend! What should I do?

Anonymous
I've been in a friends with benefits relationship with a guy for six months. After several time hooked up, we decided to start a friends with benefits relationship, and he said "You're not only for benefit but you're my friends too" I used to ask him to make sure that he's not in relationship, and even told him before that if he has girlfriend we have to finish this relationship. Cuz my previous relationship ended after I found out my ex cheated on me, so basically I know exactly how it feels to be cheated on by your partner. I kinda forgot what did I ask him, but I remember he said "every time I be in a relationship it didn't end well "

We are quite open to talk about sex, and he's mentioned that he really likes to have sex with me and want to see me quite often, so am I! And I noticed that he has developed feelings for me, he flirted me!.

I'd say that we're quite close that we bath, cuddling, dancing together, he picks me up and drops me home every time, but we never go on a date together, and never talk about one's feeling before even though I like him and I want more than that.

Last time we met he said he liked me again, so I asked him what do you like about me the most, I mean which part of me, and he said "oh it's so hard to answer because I like everything about you, I like your face, your smile, your teeth and everything" and kissed and hugged me more often. I'm trying to play cool because I don't want to lose this relationship, and even I like him so much, but I still not ready, I'm not afraid of commitment but just not ready for that.

But although I noticed that he's developed feelings for me, I also accidentally found out that HE HAS A GIRLFRIEND! when he's in bathroom I saw his gf's calling (the contact name is "my love") I was so shocked but didn't asked him, she called four or five times, I told him to pick up.

Should I tell him I know he lied about his girlfriend and I want to stop this relationship, but we can still be friends, or I don't have to mention about his gf?
I just found out that my friends with benefits has a girlfriend! What should I do?
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