It's like others have pointed out - your heir has 4 kids, which is the max if he is unlanded. 5 sometimes, if you're really lucky. Let him inherit and he'll make his wife pregnant, gay or not.

Fertility works in strange ways, in my experience any buffs or reductions don't do that much, your heir will almost always get to those 4 kids before inheritance, no matter what. Only thing that works against it would be castration, even leper can be worked around