I don’t disagree with anything you said, but I’d like to add:
It seems to me that James Franco is the problem. He’s the one making a deal of the fact that she left. He’s the one claiming she was offended. But she just quietly left. There’s no indication she stormed off or caused a scene other than his accounting of the incident. Given his many blatant displays of misogyny, his interpretation should have been suspect from the start.
Was she offended? Just because he thinks that’s why she left doesn’t mean that’s why she left.
Maybe she was just trying to preserve scene integrity. I don’t know anything about the movie or the scene they were filming, I’m just speculating. She may have needed to leave to preserve focus during filming over difficult dialogue or blocking. Maybe there were quick changes at the last minute that she was trying to visualize through and his dancing was a distraction. Maybe the upcoming scene was one where she had to cry or was romantic where the last thing you want in your head is orange speedo and boom chicka mau mau. You know? Maybe the director asked her to make sure to get it on the first take because all the antics of the funtime bro club were putting production behind schedule. Maybe she got a call she wanted/needed to answer.
It’s bias and lack of respect of another sort that your first impression of the situation was that she was offended. There are numerous examples of male actors who do weird things on set all the time to keep focus, from ignoring people who’s characters are their character’s antagonist, to demanding silence on set 5 minutes before filming, to calling people names, to cracking jokes before certain types of scenes, etc. It’s always accepted as part of their process to get into or stay in character. In fact, assuming the upcoming scene was a difficult one, it’s not outside reason that Tatum was just trying to relax the mood and unwind before filming.
Why is it shocking that actresses might also need to preserve focus, visualize, mentally map changes, or might not want to end up ruining or delaying filming because an orange speedo gave them a case of the giggles, to stay in character. Why is the assumption that they were offended because they chose to not engage and give you their attention?
To me, this seems more like another example of Franco’s misogyny rather than anything about Watson or Tatum. Because he thinks her leaving is a problem because she chose something else over giving focus to an attention seeking man. Newsflash to James Franco: not everything is about you and your perceptions are highly flawed, bro.