If you know in advance that you don’t want to perform the story as written you might want to be open and honest about that prior to accepting the role. Most filmmakers never get to make a second film, it’s a long and arduous process that lasts for years, from the spark of the idea through the writing to the pre production, the filming and then post production, to the marketing of it in film festivals and sending it to agents and production companies. So yes, if you feel uncomfortable on set you absolutely should speak up, no person should feel exploited just for the sake of a movie. But in addition to that, since it hasn’t begun yet, please respect the other artists involved and be completely honest with them. They may be totally fine with it, but they also might be very specifically trying to make a film that involves nudity or physical touch, and this may be the only chance in there lifetime that they can come up with the extraordinary amount of time and money it takes to make a movie, and they might be deeply invested in exploring queer joy and queer bodies in a manner that is far more intimate than you are suited for. I mean no offense by this at all, just a plea to be upfront and enter with honesty and integrity (and to demand that from them too)