Beginning in junior high I had a strict policy to simply not answer this question. Like many other children in families like mine, having to answer that question feels like putting our parents on trial. (See earlier question for more on this.)
In recent years, however, I have decided to let people know that I identify as straight. I think both gay and straight people need to know that it is OK for straight people to be outspoken about gay rights because they are human rights. Also, I want homophobic people to know that caring about gay issues does not make a straight person “turn” gay.