Because it's not common among humans and people find it disgusting/weird. This is in addition to religious reasons.
Though isn't it funny... to truly understand the human mind, countless amount of scenarios have to exist. Diversity in society isn't an enemy to humanity, It is humanity. Socially engineered societies and ostracism reflects a refusal to learn
about ourselves.
No one has to be the evil scientists that puts their subjects into unwanted circumstances, or questionable circumstances. Nature does that all on its own. For much of human nature, we don't create the experiment, we observe it.
Humanity isn't just you, it isn't just me, it isn't even just society, it's a bunch of numbers that can be added or subtracted in so many ways. It is letters that represents many variables. A complicated problem even for the brightest of mathemeticians.
We haven't even cracked the human conscious yet.
If everyone in society was forced to love exactly the same thing, live exactly the same lifestyles, then true evolution would come to a halt.
More than any other species, we have the opportunity to understand ourselves and everyone around us. But, people squander it with hate and prejudice.
Diversity is beautiful. I'm beginning to look at another human and see an outcome out of so many possibilities. Be it from their environment or genes.
Each difference we see in society is an opportunity to learn.