They want to be the center of attention. I'd also reckon that some do this for the thrill of telling people they're transphobic if they say that these posts don't belong in the sub. I'm actually very surprised that r/bisexual is only now being targeted.

It's similar to something someone else mentioned before: If sci-fi fans of Star Wars and Star Trek started posting in magical fantasy subs like Lord of the Rings, Harry Potter, and Avatar the Last Air Bender you'd expect those posts to be taken down as per the subs' rules. However, instead of taking down those posts, the mods say, "Sci-fi content is welcome here". So now eveyone who says, "This isn't the sub for that content," gets banned for being insensetive to the feelings of sci-fi people.

They're able to take over specific groups so easily because they're following a very simple method: divide and conquer. You have group A (the T's) and group B (The LGB's). The loudest of group A have convinced the loudest of group B that group A's feelings and experiences are more important than everyone else's. Now you have a bigger group AB trying to convince the rest of B that A is more important. More divides, more splits, more fractures, until eventually group B becomes so small that group AB is able to convince other's that anyone left in group B is evil. Which leads us to where we are now, exiled to Saidit.

It's not so bad though. I quite like the level-headedness of the community here. It's like a very refreshing oasis where my mind doesn't have to jump through so many hoops to try and make sense out of what people say. You're actually able to ask questions and learn things here (things that follow logic) without being attacked for not knowing or not understanding something. We don't all agree on everything and that is how we're able to have civil discourse. Intellectual diversity seems to be valued a lot more by the people here than by most people on reddit.

There isn't much to be done about reclaiming subs on reddit. Unless mods change, nothing will work. Something that might work though is if we all go out of our way to politely and civily state our opinions and the facts that support those opinions. This will create waves and anger some people but, this tactic is for the benefit of fence sitters or lurkers who have only ever been exposed to one side of this gender debate.