Following up, I have found out that if you use the Copy X item, you can change your characters fertility while remaining human, at least until you close the game.
To replicate this, use the Copy X item on one of your girls to become that species. Then use Copy X on yourself. Congrats, you now have fertility levels of that species as a human. It's fun to do this as a human and copy the rabbit species so that you have basically permanent 200% fertility, which also solves my previous issue in a roundabout way. Although 75% fertility is still my characters baseline for whatever reason.