What do you do when you find out that your boyfriend is gay?

Accept them, tell them you don't care and treat them like you always have. If you are truly friends, their sexuality should not affect it. Them being gay requires you to do nothing. Friendship isn't about sexuality. Did your friend suddenly grow a horn out of their forehead and their eyes bulged out? You do nothing because nothing has changed with the exception you know they are gay. I have many gay friends and they aren't any different than my straight friends. Learn to be different and accept good people for who they are, not what they are.