My boyfriend and I had a threesome but I regret it

My boyfriend and I recently spent a night having a threesome with a girl he knew. Although at the time I didn't notice, a lot of the action was happening between the two of them. I know that since he has been corresponding with the girl and I'm sure they have been enjoying sexual fun because he calls her name when we're having sex. I don't know how to handle this. Is it my fault for consenting to the threesome? Or is it his for allowing it to escalate? What do I do?

lmao..I agree with him this time^

trace the line and tell him you wouldn't want a threesome anymore... she can get her own boyfriend. if you continue to let her come with your boyfriend, in no time he'll be hers