In my experience with the local gay scene, it’s common for stable, long term relationships to be open. Maybe this fits what you said about “tight-knit pro-open communities” but it’s not like we intentionally self-selected to combine poly people together. Maybe it’s some difference in relationship dynamics related to gender expectations.
As a gay, I think that we actually have a lot of self-exceptionalism around the abuse of younger men by poly/open men in legal marriages with their primary. There's a lot of solidified marriages around fucking younger men and having a mortgage with the old same-sex ball and chain. I think it's a sort of normalized abuse we haven't even begun to reckon with.
https://imgur.com/a/internet-DWzJ26B
Anyone can make images and video now.