You'd have to edit the attack animation and everything as well, because the mouth and the vagina are not connected. At all.
Haha, it's a fantasy world and she's literally carrying a magical wand...how is fantastical biology any different? Human cross species impregnation isn't possible in reality, so unless you want full scientific accuracy, there's gonna be some unrealistic elements to a sexual fantasy.
But, that being said, if you wanted an actual scientific explanation to suspend belief:
The impregnator has developed a unique form of reproduction. Instead of wastefully killing the host with eggs, it evolved a specialized parasitical fluid. When ingested, said fluid is absorbed into her body via the bloodstream, changing her biology and causing her eggs to become fertilized from within. Shortly after, she gives birth to a new parasite, which then repeats the process, allowing multiple births.