In California, where it was legal for a short while, someone in the gay community will sometimes ask whether the couple got married during that window. It's essentially the same question as "Is it legal?"--but not quite. So it does happen. But I don' t think I've heard any straight people ask. It's all in the tribe, which means that we're not questioning the validity of the commitment but wondering whether CA will ever finally grant full status to people who married then.
I used to think about how to answer when I was thinking that my partner and I might stay together. I came up with a few answers, although I don't remember all of them now. The trick is to answer in a way that satisfies the other person's curiosity or shuts them down, without your coming right out and lying or outing yourself. I figured that if a straight person asked, I could always say, "You know, that question is a real no-no in a lot of gay circles." Another answer was of the "Does it matter?" variety.
I figured that if anyone asked when we'd gotten married, I could fortunately say 2004 because Massachusetts started legalizing gay marriage that year. I could always say that or state that my marriage is legal in Massachusetts (which it would be, given the situation there) and let the questioner assume that we had flown to Boston or something. The only trouble with this response is that it's very close to lying. But I'm sure that a little brainstorming will give you a good creative answer.