As you well know, it is usually impossible to prove a negative, especially when it supposedly occurred in a private setting between an accuser and an accused.

> The stories of the women involved seemed very credible, and unlikely to have been made up, considering they were supporters of him.

They did not seem very credible. That is, it is credible that they encountered Assange, but the accusations not so much. At any rate, they were not credible enough for the Swedish authorities to decide to indict Assange (which they could have done).

> In any case, Assange is a controversial figure. ... I am concerned by the rape stories. I agree he's being made an example of for political reasons ... but that doesn't ... mean he's entirely innocent

Assange's current imprisonment is not about any allegations of sexual misconduct. He is not accused of any of that. Also, be aware that Assange was _made_ controversial by the treatment of those allegations and by the UK's insistence to put Assange in custody. Yet we know that the UK was motivated not by the Swedish affair but by the US, and went as far as to pressure Sweden not to drop their extradition request. So, regardless of whether you are skeptical of Assange's personal/sexual conduct - that matter was cynically used to persecute him politically.

And thus IMO you too have been manipulated to effectively support Assange's ordeal, including the extradition possibility, because of your - understandable - concern for possible victims of sexual misconduct.