As everyone else has said, low sex drive or loss of interest doesn't mean he's gay. My boyfriend has had no interest in sex with me for several years (but that's another topic and issue altogether), but gay is one thing I have no inkling that he might be. Is there something else that makes you think he's gay?

After being married (or just together) for a while, it's not unusual for one or both partners to have dwindling sexual interest. You might want to see a counselor (or a doctor, should it be a possible medical issue, as Sloth said), and try some ideas for spicing up the marriage.