Tieflings and Aasimar are infertile. These subraces result from even a minute fraction of the blood from the Pits, so pervasive is the heritage... so just as half-elves can no longer breed with either elves or humans (or half-elves), Tieflings and Aasimar can no longer breed with straight humans.(Also, Half-elf + elf, in Layonara != elf.)
Oh, but a 3/4 human 1/4 elf is Okay to make babies?