I think it is extremely hard for kids to come out to their parents. They know so many horror stories, and for most a parent is someone they don't just want approval from, but need it. So despite how I thought my kids would know that we would accept them as they were, it turns out our second child did not know, and intentionally hid a lot from us. Until my grown child learned enough self-confidence to stop caring if we approved, the risk was unthinkable to them.
I think the best you can do is continue to project a caring attitude, openly speak about not understanding how any parent could ever reject their own child, and so on. Don't share your worry about gay couples not providing grandchildren; any little thing can drive a scared child to believe they will profoundly disappoint their parents if the parents learn their truth. It is so so very hard for them to tell their parents.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).