I attribute the problem slightly differently--in that it's that people are used to using "partner" for same-sex couples and "husband/wife" for opposite-sex couples, and so, instinctually, they realize I'm queer, and assign "partner" as the appropriate. I don't think it's malicious. I think it's just a sign of the fact that people have different words for these different classes of relationships stuck in their head, and those habits are hard to break out of.

Now, the dude who I got my MA with, who, when I saw him at a conference, insisted that I referred to my wife as "my husband" at one point in time...that was another issue.