I recall some years ago when I met another bl friend for the first time. He was my best friend from online and we were finally meeting in person for the first time. A mutual friend (also BL), who was hosting this meeting, took us to the gay district of a major metropolitan city. My friend was amazed that the gay mannerisms were so prevalent. Apparently, he had been under the impression that swishing is a stereotype and real gay people aren't like that.

That has nothing to do with what you asked, but I was reminded of it and thought I would share.

Now as for the question of where mannerisms come from, I have no idea. I have wondered the same thing but never cared enough to research it. The only evidence I have is anecdotal evidence from parents who claim that their child inherited mannerisms and peculiarities from a father they never met. This would hint at a genetic factor. However, this is far from reliable evidence; it's just as likely that it says more about the mother than the child.

It seems to me that mannerisms are a social construct, whereas sexual orientation is hardwired in the brain. This being the case, it would follow that sexual orientation would come first.