You see it more on tumblr when Authors share anon asks they get. It gets really bad to the point they shut off asks because of it. It often comes in the form of whining that the reader can’t romance X character that has set sexuality. Sadly the most hit targets of this seem to be ROs that are aspec (from what I have seen). Woe be any author who makes an attractive aspec RO. The upset that rains down is constant. Offhand I know When Twilight Strikes gets hit A LOT for B and K as ROs. But I’ve seen it hit in some degree all sexualities. Some turning gay and straight characters bi under pressure to please readers. And some authors just simply breaking down from the onslaught.
You also get set NB characters getting this treatment, less about change and more about finding out information in order for readers to designate them a gender rather than leaving them happily NB and accepting them for who they are.
I think you maybe see it less on the forums because people can’t hide their identities behind anon for these topics. The issue on the forums get skirted around, it’s subtle though, enough that it could very easily be me reading into it too much. With gender issues, you see more trans character misgendering and upset on the forums than the other issues. There is much upset when an RO is trans because people decide they don’t count as X gender romance because trans or feel transpersons need warning labels.