I am hard pressed to find a credible argument that shows me how two people who have been married, had kids even, then divorce - doesn't threaten the institution of marriage more than the idea of two men or two women who happen to be in a committed loving relationship who want to marry. The current rate of divorce among the heterosexual population is more of a threat to the institution of marriage than any gay marriage would ever be. Those gay or lesbian couples who actually went through with marriage, are the ones who have been in long-term established relationships and have just as much of a chance if not dare I say maybe even a better chance of staying together than their heterosexual counterparts. So, where is the threat?
And, don't even come at me with the procreation thing! What about the heterosexual couples who are infertile- or chose not to have children at all. They still get the special privilege of marriage. Why? I know of quite a few gay and lesbian couples who have chosen to raise beautiful, bright and well adapted children. I think that puts them in that procreation category.
And don't come at me with Leviticus either- you don't get to pick and chose which parts you follow and which ones you won't. I mean we could always re-institute some of the other ones if you insist on hanging on to this one - maybe the stoning for blaspheme - yea, that's a good one...