I say let them ask on their own.

I would prefer not to expose them to the idea at all, but with acceptance of it being rammed down everyone's throat at every turn these days, it's hard to avoid. Not only that, but the push back against that is breading hate toward homosexuals. So as a fairly conservative, christian, parent, I have to try to explain to a young child how both sides of this are wrong. While homosexuality is sinful, and we are right to hate the sin, like any other, it's also sinful for us to hate those people because we are no better.