User blog comment:Tardir/New Species. Yaay./@comment-2253300-20110417160711

Actually I read somewhere that human and Eldar are similar enough to interbreed. I'm not sure where I read it, but it was probably either Lexicanum or the 40k wiki. Regardless of that, NoFury is right. Eldar and Orks are simply too different genetically to reproduce, especially since the Orks don't reproduce in a sexual manner to begin with.