Hey, thanks so much for your input.
I actually gave this a lot of thought (I didn't realize so many people were into pregnancy stuff until this first launch), and this is what I have planned for the in-universe explanation (spoilers):
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Human-anthro reproduction is blocked by fundamental genetic incompatibility. During evolution, anthro species developed different chromosomal structures, hormone receptors, and reproductive systems.
However, there exists an extremely rare alchemical catalyst that can temporarily bridge this genetic gap. A rare flower called an Ethereal Lily can be turned into a potion by any apothecary that, theoretically, can lead to cross-breeding between humans and anthros.
Although the game isn't going to be ending anytime soon, when eventually there comes the time where I am wrapping up the game, I will likely have a few different endings. One of which may include hunting this flower down and drinking it and then you can get (likely Lily) pregnant and start a family.