
Laying my cards on the table upfront here: Joss has always reminded me of an ex who talked a good game about feminism, while he was cheating on me and lying to me about it. I believe Kai Cole. If you think that's because I'm letting my personal experiences carry too much weight in this matter, that's your prerogative.
I think I'd have a much easier time "separating art from the artist's personal life" if Joss had done the same thing. If instead of sleeping with actresses and directors and writers, he'd been sleeping with execs at unrelated Fortune 500 companies. It still would have been a crummy thing to do to his wife, but it wouldn't have left me wondering how many character arc/plot/casting decisions on the show were made because someone would or wouldn't sleep with Joss.
As to whether Kai should have come forward: the affairs involved at least the possibility of abuse of power in the workplace, and I think it's a good thing for future collaborators to know about Joss so they can make a fully-informed decision for themselves whether they want to work with him. And because our world is kind of crummy, the only person who can speak up about Joss' predilections without career blowback is someone like Kai, who works in a completely different industry.
Other things I've heard people say in response to the story: "This kind of stuff is really common in the entertainment industry." When someone like Joss holds himself out as a feminist, he's telling the world that he is trying to be different than the status quo. Feminism is making changes (and sometimes sacrifices) in the interest of making the world a fairer, more equitable world for women. Joss cannot change the entertainment industry by sticking to the old sexist behavior (like the casting couch). Joss has to try to be better than the status quo, if his feminism is something real, and not just a story he tells about himself in the manner of Andrew in "Storyteller."