3some gone wrong!!!

I had a 3 some with my bestfriend and her boyfriend and now all she wants to do is have sex with me!!!!...she's my bestfriend and I love her to death but I don't wanna keep fucking her...everytime I spend the night at her house and the lights go off she wanna have sex....and I never want to....but I don't tell her no I just do it.....I don't wanna lose our friendship and hurt her feelings...HELP ME YALL GIMME SOME ADVICE!!!!