Guy's Behavior

I've slept with his best friend?

Hey guys, please don't judge me..
I'm almost 22 years old, and i moved out to a small town (400 people).
Met a guy, he's 18 (that's legal here lol) and we became good friends and had sex, we decided that we're friends with benefits. No feeling, just sex.
i've been with only two man in my whole life (both in a long term relationship). One night i fall asleep at his aprtment and he woke me up and asked me to leave, i felt so bad. He decided to cut it off cause he wanted to sleep with other girls and said that he had the best time with me and that we had an amazing sex, but he can't have sez just with me. I agreed and told him that i will have sex with other guys and it might happen with his friends (small town, just a remainder) he said "you can f whoever you'd like, even my best friends, i don't care, i love you as a friend and i'll be happy for you, and i'll tell you about my nightstands and ask for your opinion on stuff etc.." he huged me and smiled to me and when i left i cried my eyes out.
Few days later his best friend (which i didn't knew that he's his best friend) came to town and we all been at the pub dancing and drinking.. i had sex that night with his boyfriend.
The next day he told me that his boyfriend came to ask for condoms and he asked him "we're going to have sex, are you ok with that?" And he said "do whatever you want, just know that i'll have a problem with her latre on". I didn't knew that his boyfriend knew about us at all so that's mean he knew and still fucked me. What a shitty friend.
Anyway, the day after he told me that he hates me and that i'm a slut, wtf? He said that i can f whoever i want, what does that mean?
I've slept with his best friend?
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