I've done some work that involves interviewing people about their sexuality. Here's my advice, for whatever it's worth.
Mostly people never have the chance to express themselves candidly, to a stranger, about their sexuality and sexual experience. If you create the right environment, it can be an incredibly validating, probabably unpresidented experience for your subjects.
Let people tell the story they want to tell, even if it seems to be pretty fair afield from your objectives, or what you think your objectives are for the film. Be patient, listen, as best you can try to understand why your subject has choosen to reveal themselves to your in such an intimate way. No one agrees to be recorded talking about such a personal subject unless they feel like they have something important to share, something that outweighs the risks. Even if they can't articulate it, your subjects will have an agenda. Figure out what it is, help them express themselves.
If your lucky, you find points of tangency or even overlap with your agenda. The interview becomes a dialog (even if only one side ever makes it to the screen) When you find that, the sparks will fly! The footage you get will be good as gold!
Good luck! It sounds like an exciting project!