The humour is just the juxtaposition of gay porn and Japanese-ey aesthetics. It originated in Japan, where I think a lot of the humour they got out of it was "mishearing" certain lines as similar sounding absurd Japanese phrases. The trend in the English speaking world I believe just picked up the absurd juxtaposition and used that. These remixes were usually posted on /f/ where they gained traction as elaborate bait-and-switch posts, which I imagine only stoked the flames. I think the best part of the entire thing is how well some of the actors in these edits took it. Not all did, of course, but certain characters like Aniki and Van really ran with the whole thing, took it in their stride and cultivated a cult fanbase (RIP Aniki). Not to mention the scenes themselves are usually so woefully poorly acted they're innately hilarious by themselves once you cut out the actual sex.