Because Baby Jesus said so (Oh, wait, he didn't...;-D...)
Dear GOP,
I'll tell you what. I'll support whatever you wish provided you do just one little thing. Take my class to get control of the Stockholm Syndrome/Zombification that has permeated your brains since before Regan got zapped in the noggin.
I will de-pervert your thinking. It is called perversion therapy. It makes you an objective human being again and stops your incessant pointing the fingers at those you cannot possibly understand.
Yes, it's a thing. Do I agree with it? No. Though is this significant? Not at all. It's just not important to the big picture. The left obsess about fringe issues that don't really matter and ignore the elephant in the room.
The reason most cultures traditionally suppress homosexuality and ANYTHING ELSE that runs counter to the traditional family unit is because that's what works. That's what creates growth and prosperity. The traditional family is the building block of society. Any society that departs from it will go into demographic collapse. It's all over for cultures like Germany. The collapse has already started.
Homosexuality in itself is just a small part of that equation. Though when a society gets liberal on one thing, it's close to getting liberal on everything. If one group gets license to do whatever they want, pretty soon EVERY OTHER group wants license to do whatever they want. In fact they don't just want tolerance, they want respect or even admiration. Society becomes narcissistic and self indulgent.
The truth is most men would rather spend their time building harems than committing to one woman. Most women will dump their man for a man of higher status in a cold minute. When humans are who they are and do what they want everything goes to sh*t.
We don't have to be authoritarian about it. Alternative lifestyles can be tolerated. Human ego is strong enough to shape society into the right structure simply by denying validation to everything except the traditional family.