Thanks for contacting Just Answer. What do you mean by "what do I do?" What do you do about what? Is this a Health question, Legal question, or something else?
How do you know he is gay?
That doesn't necessarily mean he is gay. It may mean that he has a physical problem, that he isn't interested in sex with you any more, or that he is upset about something else in your relationship. He should see his doctor to see if he has a physical problem that is affecting his sex drive. You may also want to consider marital counseling to save your marriage.