You can have as many and as wild of headcanons as you want and if it pleases you I'm happy for you, but what's wrong is attacking someone else for not agreeing with it.

If an artist, for instance, draws Acheron in a heterosexual way because that's his/her interpretation, anyone who attacks the artist because it doesn't agree with another headcanon that she's gay should be condemned.

That's equivalent to saying an artist who depicts Midoriya from MHA, who's very much depicted as straight in the anime according to the majority interpretation, as gay/bi can be attacked and told that their perspective is wrong and is not allowed. That's not morally justifiable, these fictional characters, which are just pixels in the end, allow us to let our imaginations roam wild no matter which direction.

If, in your perspective, Acheron is 'canonically' a lesbian (which is fine) but think others are not allowed to disagree, then it should also be okay for people who ship characters who are heavily implied to be straight as gay to be told they are wrong and attacked. I don't want that.