I think just warning in the post (or post title) would suffice. Something like "Movie Name - [M/F; M/M]" could work. There are situations exactly like this. It wouldn't make sense to create multiple threads, it just creates confusion and clutter.
As long as there's proper warning, it should be okay. Although I'm bi, I agree that people who don't want to see it have the right not to be surprised by it.
Also agreed. There should be a requirement to post screenshots for anything posted (if there wasn't one already). They're easy to do and would help people to decide if something is worth downloading or not.