Smile, and hug them. Let them know they aren't alone in this, and we can get through anything together. It's not easy being gay (or green, if you're Kermit), but together we can do anything.
It's happened a few times - I'm very much out, and happy to be asked anything...so people do. I figure if I can make the journey easier for just one person, then my time on this earth has been worthwhile. More than one, awesome.