There is no logical consistency except what allows the continued survival and flourishing of life and the human race.
Having sex with animals could get you sick and cause all sorts of problems. Eating animals on the other hand can extend your life beyond a few days and perhaps even into years after you can no longer drink your mother's milk and has very few downsides, especially with the invention of cooking. Sure, we don't need the source of sustenance that is meat today when we have several times more food than is necessary to feed the whole globe and then throw a lot of it away, but this wasn't true for the vast majority of our history. People only a few hundred years ago had to scrape for every protein they could find.
There's no special moral reason because we didn't decide. It's just an instinct, though one that we can examine and ignore if we want.
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