"Not liking someone" is indeed a poor excuse to believe their accuser.

But "historic record" is different. Joss is an avowed Male Feminist. That is, he heard the ideology "all men are evil rapist oppressive monsters" and thought to himself "yeah, sounds about right when thinking of myself, I'll join this movement".

The standard of evidence required to convict someone in the court of public opinion varies wildly person to person, for sure. But I think anyone who starts the proceedings with a confession that according to the ideology they adopted willingly, they themselves are the most irredeemably evil thing there is, the standard of evidence changes from "beyond reasonable doubt" to "is he protecting someone worse?".

If... I don't know... Mark Zuckerberg was accused tomorrow, I don't like him, few people seem to, but I'd give him the benefit of the doubt to wait on lots of evidence as he has not confessed to wrongdoing as far as I know for now.

But Joss preached "believe all women, believe them without any evidence, and immediately. Men like me, Joss Wheden, are all evil rapists". It would be untowards and immoral of me to go against his wishes and act contrary to his requests and ideology when judging him.

I would want to be judged based on my ideology and requests, so it is only moral for me to assume he would want to be judged based on his, too.