@Laura Baker,
I don't understand, what are you concerned about? Many gay and bisexual men have perfectly good relationships with their families. Obviously there is a strain in your relationship with you son. Why not focus on this?
I don't understand why the sex life of your adult son would concern you at all.
Let me speak with you as an adult male. I have a pretty good relationship with my mother. We talk regularly about a lot of things, work, family, plans. However, I don't ever talk about my sex life with her and she never brings it up. If my mother were worried about my sex life, I would find it off-putting (and a little strange).
If you want to be involved in the life of your adult son, work on building good communication. Ask about the things he cares about. Tell him about what you are doing. Understand that adult relationships with parents often have boundaries. For me (and I suspect for many men) respect is very important. If I feel someone is not respecting me, I am going to push them away.
But understand this. No adult man, gay or straight, wants his mother involved in (or concerned about) his sex life. If you intrude on this area of his life, it is no wonder he is pushing you away.
His sexuality doesn't matter. Just drop it. You are not going to have any say or part of his sexuality. Don't feel bad about this, this is just the way it is.
This doesn't mean that you can't work on having a healthy, meaningful respectful, supportive relationship with your son. As long as you drop whatever hangups you have about sex.