That's a good question; hopefully I can clarify what I meant.
None of those three individual pieces of information are a problem, taken as stand alone data (assuming that informing the class that he is gay wasnt done in an inappropriate way).
It's the making it personal / political that is a red flag.
When I say red flag, I don't mean 'Omg he's probably a paedophile, sack him!'. Nothing he's done suggests that at all. I am talking about political influence - taken together the things he has said are skirting around the edge of being overly political in the context of teaching a class of children.
This isn't about pre emptively censuring someone for something that they might do in future. It's about keeping an eye, based on what has been said so far.
When safeguarding failures happen it's usually because no one had an overview of each little incident that would have built up a bigger picture. On its own, a teacher noting that Billy keeps coming to school in dirty clothes isn't necessity a problem, but it will be noted and recorded as it could be part of a picture of neglect. Schools have to do this with bullying too - every little incident that on its own may not sound like anything to worry about is recorded, so that if little Johnny says he is being bullied, the school has a complete picture of all Johnny's possible negative interactions and can see any patterns developing that might not be visible to an individual teacher who only has their part of the picture.
Safeguarding procedures and appropriate boundaries are something that teachers are trained to be very aware of. If this teacher isn't, I would have concerns that he hadn't been properly trained or is deliberately prioritising his politics over his training - again, a red flag.
He's a drama teacher. He could have legitimately spoken about gay reprentation in the media in the past - because that's a historical fact. He could even have qualified or with 'some gay people felt excluded by this' etc.
Linking it to his own personal experience is what makes it too much.
In the same way, if I were a teacher, it would be acceptable for me to comment on representation of women in the media. It would be inappropriate of me to qualify this with 'as a feminist, I think that xyz' or to speak about my specific experiences and how it made me feel (unless it was a specific PSHE leading in that subject and I was qualified to teach it). Not because being a feminist is inappropriate or shameful or inherently harmful to kids of course, but because it would be putting an inappropriately personal, political slant on the information I was teaching them.