As people have (repeatedly) pointed out. Humans are cultured to harbor a strong hatred for all things alien, mutant, or arcane in the Imperium.
For a human to be cultured in any other way, he/she would have to have come from a world well outside the Imperial Influence and long arm of the Ecclesiarchy.
However, being from a world, not protected by the Imperium, probably incurs such risks as Chaos influence, Tau Influence, Rampant Orks, Nids, and possiby even Necrons... all of whome THRIVE outside Imperial Influence. (Necrons not so much thriving as... sleeping).
Moreover, Eldar harbor just as strong a hatred for humans as we do for them. Sinse Eldar are so arrogant, and self-righteous, always casting a downward gaze at the "Human Pestilence" they are so fond of calling us... it is unlikely any Eldar would suffer a relationship with a human.
Then you get into the actuall physics/genetics... Im not sure Eldar or Orks even have gender... sinse they are both products the Old Ones' experiments. They were both bio-engineered to combat the Necrons and the C'tan. Reproduction for them is likely nothing at all like human reproduction and thus its entirely possible that they simply couldn't breed with humans even IF hell froze over than they chose to.