Threesome: The Aftermath

My boyfriend and I had a threesome on New Year’s eve. We through a small get together at our house and ended up having a girl stay the night. My little sister stayed the night. His cousin (27) who was passed out from too much liquor stayed over as well. (We did not have a threesome with my little sister and the cousin lol they just were at the house too. The next morning my boyfriend and I swore that what happened would only stay within the apartment.)

Fast forward to yesterday. We were at his aunts house and she says “I heard about y’all and ____” and we were like what are you talking about? So we ask his cousin and he says that he told her that we had a threesome. (Also the aunt is not either of their moms)

Now I’m livid because why did he think that was his business to tell? Especially about my sex life. I was so embarrassed. Now my boyfriend is telling me that I am dragging it out and his cousin met no harm by it.

Do I have a right to be mad? What do y’all think about the situation?

UPDATE:

Thanks everybody for your honest opinions. I talked to my boyfriend and advised him I’m done interacting with that cousin because I don’t feel like it was right of him to tell business that wasn’t his. I’m also very embarrassed so I’m done going to that aunts house too. Although we did have a threesome while they were here I don’t feel like that gives his cousin the power to tell anyone. I feel violated. To each it’s own though. Thanks again!