I (22M) dont want my gf(37F) playing sexually with her best friend (35M). Am I being controlling and possessive? AITAH?

I (22M) am not comfortable with my gf's(37F) and her bestfriends (35M) relationship, how do I go about bringing this up with it devolving into a argument

I (22m) don't want my girlfriend (37) having any form of sexual relationship with her best friend (34m) but she says I was being controlling and possessive. Am I being unreasonable?