Board Thread:Warhammer 40k General Discussion/@comment-7699236-20130626164907/@comment-1583177-20130626205943

The only way a half human half Eldar could exist if the Eldar weren't a subspecies of Homo sapiens would be if they were gentic engineering, and given that the only people who know how to do this sort of stuff are machanicus adepts (who are taught to abhor the alien as their first lession) itd be very hard to actually create half eldar half humans this way.

However as I said, if we decide for a moment that Eldar belong to the homo sapiens family (which actually has some semi-canon backing in the old rogue trader days), they're would be no problem provided nobody wanted the half Eldar to become a space marine, or then get pregnant / father another child.

Anyway regarding Inter-xenos relationships, most of the time phenomenally they would repel each other, and the vast majority of the exceptions are either extremely one sided (a human male falling for an eldar female who would slit his throat as soon as look at him) or are 1d4chan fanfics (like Love can bloom and the idea of Lofn, the illegitimate child of A vindicre assassin and Farseer Taldeer).