I have two boys and I just wouldn't care. I let them know all the time that I am there for them no matter what, apart from my family the only people I let look after them are a couple that consist of 2 men, so my eldest who is 6 already has very strong opinions on what is not ok in describing a gay couple.
I don't think I would act shocked, Id ask them/him if they needed to talk about it but if not that would be the end of it, nothing more said. I would try and guide my son into just being who he is and labeling himself by his sexuality. I really think its about time where sexuality shouldn't be an issue and no presumptions should be made that a person is heterosexual. I hear all to often, they lied about their sexuality, when in fact they haven't, they just haven't disclosed their sexual preference to that person.
I wouldn't have any interest in what my sons ever did in the bedroom (or where ever) with anyone, male or female. So as long as the are happy and feel loved and respected thats where my concerns stop.