[Mentathiel]

Well, if you look at elven racial armours, all three elven races (high, wood and dark) create armour which can be worn by half-elves. This suggests that all three races spawn half-elves regularly enough that they have made their armour adjustable for their half-human Ayr'Dal children.

We know that elves are genetically compatible with other races too; in the Neriak rogue's guild, there is a quest to murder a pregnant halfling because she is carrying a dark elf's child.

But, of course, consider that even the wood elves call them Ayr'Dal, a word meaning outcast, mongrel or misfit elves. It's not always racial purity or xenophobia, just a tendency for pure-blooded elves to be considered 'normal' and anything else is therefore deviant.