I do not, and will never recognize any marriage other than those consisting of one man and one woman.
I wish conservatives would do a better job of articulating the real issue, which is not whether groups that do not contain exactly one man(*) and exactly one woman should be allowed to regard themselves as "married", but rather whether such groups should be able to use state power to compel others to recognize their union. If a guy wants to declare that he is the wife of his male friend, dog, his Jeep, and/or the number 57, that's not my concern, but if he wants to force me to recognize his marriage, then that is my concern.
Putting things in such terms makes clear, IMHO, what these people's real agenda is. Incidentally, if "civil unions" aren't about sex, why should civil unions not be permitted among siblings or other relatives? In some cases it might actually make sense for a brother and sister to form a civil union (e.g. if both are widowed and have children, it may be better for the brother's children to be raised by their father and their aunt, and for the sister's children to be raised by their mother and their uncle, than for the widowed siblings to raise the children separately. In such a situation, a "civil union" would seem entirely reasonable (so as to establish joint ownership of property, facilitate custody of children in case one sibling gets killed or incapacitated, etc.); why should the fact that the people are siblings count against it?