#9 Futa: could have a bug/parisite that attaches to the clit. The idea come from a Ero Dungeons mod, but is as it sounds - it attaches, and "rewards" the player by stimulating them when ever the player penetrates someone/something it can lay eggs in/ impreg.
Additionally you could even introduce the enemy by being impreg by someone else in game using the method above. (I'd picture the "birth" leading to attachment).