While I do detest it when people claiming to be "open minded" are, in fact, extremely close minded, I do not consider this case to be a good example.
Open mindedness is about how much you're willing to change your views when confronted with evidence and strong reasoning conflicting with what you believe in, and the contents of those views actually has very little to do with it.
In the example you gave, both sides could be open or closed minded, their stances on gay marriage is not important for that distinction, the distinction comes on how much they're open to changing their mind about such a thing.
Incidentally, I don't think open-mindedness is always a good quality to have, especially not on all issues. I regard myself to be close-minded on many issues, and am perfectly happy with that.







