I'm sure there were rules for playing monsters as a player class and/or taking levels in their Monster class (i.e. Becoming more representive of their race).

Now, maybe looking at a book like the *Book of Erotic Fantasy* (I belive that was that book's name...but, what it did was add`like rules for love and pregnancy) might give some insight into how conception might occur.

Ok,
*The Book of Erotic Fantasy p49-50*
>CrossBreeding:
...sex between different species happens with unusual frequency.  Creatures of the same type (humanoid, monsterous humanoid, etc.) are able to produce children, though this progeny is a half-breed, with traits from both parents...




One thought would be the Priest appealing to his/her deity...
Also, there is no explicit rule stating that said races cannot conceive (only the rarity of said conceptions not generating an explicit entry in a rule book)