There is no answer with the information you've given, but I do have a few good reasons "why."
THE GOOD
1. Older men loose the desire to have sex, it is not anyones fault, it is the chemistry of the body.
2. Some men get/have erectile dysfunctions; sometimes they are too embarrassed to tell their spouses and they are too embarrassed to get medical help.
THE BAD
1. Lack of sexual/physical attraction...
2. Anger and manipulation - just like women withhold sex as a punishment or use it as a tool to get what they want, so do some men.
THE UGLY
1. Another woman - often men who are cheating will not desire sex with their spouse because it makes them feel guilty.
2. Moving on - sometimes men who are ready for or planning a divorce will stop having sex with their spouse so that they can start moving on emotionally.
No offense, but "my husband must be gay," sounds like a cop-out to me. If you are seriously thinking that, then either you don't know the guy yet you married him anyway, or you are looking for an excuse...
I'm assuming that's not the case.
More often than not its just a difference of sexual needs; you need more, he needs less. This seems to happen especially when the husband is older than the wife. Honestly, it seems to me that men have a finite amount of sexual desire, they seem to blow it all when their in the 18-25 range and then it falls off almost completely when they hit 30ish. Women on the other hand tend to have a more steady sexual desire throughout their lives...
Communication is the foundation of a good relationship. Talk to him, tell him what's going on and hopefully you can find a happy solution/compromise.
-- A recent poll found that most married couples only have sex once or twice a month...