I always thought that the retort used by gays that divorce ruins the sanctity of marriage more than gay nuptials a poor one. Divorce doesn't affect the sanctity of marriage at all. What does divorce even have to do with the sanctity of marriage as an institution at all? It may ruin the sanctity of your invidual marriage, but hardly says anything about the institution itself.
However, from a Christian's point of view, it would violate the sanctity of marrage because in their view, your redefining what marriage is in the first place.
I'm gay, but come up with a better argument. For realz.