To expand on what Charlotte asked you is it because you don't see the extra vagina as a threat? That it isn't real sex? And you feel that another penis will more likely lure her away?
We have a name for this, it is called 'One Penis Policy' and a feature of male entitlement that IS sexist and it IS rooted in ownership of women, so although I too, do not think you are being a jerk, this is rooted deeper than a boundary, this is to do with your real views about women and female sexuality i.e. women are something you and your gf can 'share' (as if she is a dessert or some inanimate object) but men are 'real' people.
Do you understand why this might come across as sexist?