I reckon that it's way way easier to perceive her being with a guy as a degree of cheating, while doing it with a girl would be seen as more...well,recreational.The idea of having a threesome with two girls, well i would be able to enjoy that i think, i wouldnt have to shag this other girl, just my own girlfriend. It wouldnt hurt seeing my girlfriend having sex with another girl. But a threesome with another guy being inside my girlfriend, i would never be able to do that, it would ruin the relationship for me. Since that makes me sound like a hypocrite i have always considered all threesome's a no-go and didnt want to share or be shared. A sentiment my girlfriend always said she agreed with.
So you'd might want to consider re-evaluating your position.