Why is it that people want to know whether an actor is gay or not?

The biggest reason isn't really all that deep. Some people just tend to be curious about other people, especially celebrities. It isn't much different than watching all the latest news on the Kardashians, digging into if an actor is cheating on his wife, or talking endlessly on how many more kids an actress is going to adopt. It is just juicy gossip.

Some people feel that being homosexual is wrong. They want to know if supporting that actor aligns with their personal beliefs.

There are also other reasons. Sometimes people may be realizing they are gay and want to come out. When a celebrated athlete, like Tom Daley, comes out and says he is gay and happy with his life, it gives them some hope that they are not alone in what they are going through. Seeing that some celebrities that are open about their orientation, but still appreciated by their community, gives them hope that they can be too.

Even people that are already openly gay see actors and actresses coming out as a another step towards equal basic rights.

Others may want to know for completely other reasons.