More like they don't have all the things necessary to reproduce, not that they aren't being allowed. You kinda need eggs and sperm. That's why gay couples and lesbian couples sometimes get together to have kids. They're all gay, but they can reproduce that way.

And here's a conundrum. If a horse mates with a donkey, they create a mule, a retarded, infertile mule. Why would a higher being allow that abomination to happen?