Well I guess you understand the Desire System if not it is explained in the Game. Aside from that, every Enemy Type has specific Scenes that they can start. Nerds as an example only can start the Footjob Scene. When it comes to Enemy Types that can start Vaginal Sex, we talk about Thugs, Lizardmen and Guards. Goblins can transition their Scene to Vaginal Sex. Probably missing one but that is the short of it. Also important for Vaginal Sex, Karryn must be wet and her Vagina must be accessable for enemies. Also Note worthy, almost all Enemy Types can counter a Kick of Karryn for Vaginal Sex, if all Conditions are met.
Oh and obviously a lot of different Scenes others can join in into Vaginal Sex. For that, Enemy Type doesn't really matter.