Trans people have always been major players in the LGBT acceptance movement, though there are some LGs willing to throw trans/bi people under the bus for their own rights. There's a lot of infighting. But there is some truth in saying that the LGBT movement is strengthened by having more people going to bat for you. No one, and I mean NO ONE, is happy that pedos are trying to claim the LGBT movement as theirs.
On some level, it is pretty offensive that poly people feel the need to "come out" like a gay person might, but there are a decent number of gay people who are also poly. There's also the fact that if, say, they were in a closed triad pursuing the building of a family, sooner or later you'll have to explain to your family why your kids call both Jenny and Sarah "mom" when you originally said Jenny was just a roommate. On the other hand, most people in closed, committed triads don't post on r/polyamory.