I have experiances like this with my gf.
what i do, is I agree with her. here's how our last one went
her: omg babe, you're friend (so and so) is so cute
me: i know yea? i mean seriously. i tap that every night.
her: wait... say that again?
me: I had sex with him last night. it was really hot and sweaty
her: XD (never talks about him again)

you see, if you stay comidic about it, things work out well lol