AIO GF Wants 3 Way

25M here and have been with my GF for 5 years. I’ve been noticing over the past year my GF and her girl friends will get touchy and make out when they’re drunk. Idk if it’s just me but I’m not bothered by it, if anything I think it’s hot. What really threw me off is when my GF expressed interest in having a 3 way with another girl. The selfish male part of me wants to say yes but I don’t think it’s going to end up well in the long run. I imagine in the future my GF saying something along the lines of “well we did it with a girl, let’s get another guy in on it.” I expressed this concern to my GF but she assured me she has no interest but at the end of the conversation I said I would rather avoid the whole situation entirely as I don’t want to wreck our relationship. I also don’t think it’s fair to her to only have other women in the 3 way but not other men. I just can’t take the thought of another man with my GF. Anyone have a similar experience?