How was I supposed to know?

Today I received some startling news - a guy I had known since a few years turns out to be gay. I'm not giving his name out here, some of you may be able to guess!
I didn't really know him all that well; more of a "hi-bye" kinda thing. I would meet him at all the basketball tournaments. But whoa! I'm surprised!!
Anyways, it explains a lot. How even though he always had this stud-like image and swagger, he was never really ever associated with any particular girl. And it's not as if he had to try - today when our girls' basketball team heard about it, they were all like, "Oh, what a waste!"
It also explains a lot of other smaller things that I shall not bother to mention here, but I do have a question to pose -
What if your best friend, of the same sex of course, suddenly turns out to be gay? What would you do?
Awaiting some replies...
(Sounds like the stuff one could make a movie on!)