Why is Gay Sex forbidden?

I am not trying to be rude, I am simply curious.

I am aware that gay sex is forbidden, but my question is why? Incest, Bestiality, Adultery, all have practical reasons for being forbidden, but I am wondering what the reason behind gay sex being forbidden is. I come from a reform background and I have many LGBTQ+ friends and family, and I am simply wondering why? Is the reason simply G-d said so? Once again, I am not trying to be rude or condescending in any way, I simply want to know.