The majority of this community doesn't like romance in OPM.

As soon as you go to any other platform - twitter, tumblr, pixiv - it's everywhere.

Also, according to subreddit surveys, 90% of the subscribers to this sub are (presumably mostly straight) dudes. A ship between two dudes doesn't really appeal to that kind of audience in the same way het ships do.

Additionally... Sometimes folks here can be just kinda mean and hold double standards for queer content. It's gotten better but the disparity still exists.

Regarding your second point - apparently Genos' personality hasn't actually changed that much from before he was cyborgified. According to ONE, he was always a very intense kid. That's not to say he isn't without his trauma; the fact that (manga 150 spoiler) >!detonating his core still crosses his mind<! means that he still has a long way to go for expressing himself healthily.

Regarding point 3 - you're probably right but also - all ships are valid because no ships are valid. I always hope we can just support one another making fan content and having fun without bothering to point out that someone else's fun is "inferior".