1001+ ways to screw with your DM/Party

[illyahr]

Or you could assume that, just like there are spells to cure disease, heal injuries, throw fire, fly, teleport, turn invisible, stop time, bring back the dead, travel between dimensions, and so forth, there's a low-level spell that acts as a contraceptive (even without checking, I'm certain BoEF has such a spell)

It's called Block the Seed and it's a 1st-level arcane AND divine spell.