Dr. Ronald Sider in an interview with the Village Voice explains why he signed the Manhattan Declaration. I thought this was a really good question and an excellent response:

RS: How do you feel about gay people wanting to live by these kinds of conservative principles in marriage? Isn’t the desire for gay people to get married, build a life together, buy a house, raise some kids — isn’t that kind of a vindication of the values you promote?
It’s better for the people involved, and better for the culture, if a gay person has one longer-term relationship than a whole bunch of temporary ones and promiscuity. It’s pretty clear that that’s a destructive way to live. I’m glad if a gay person has one longer term relationship, rather than a bunch of relationships. I don’t think the culture needs to say that partnership is marriage. I think it would be entirely appropriate and there is a range of views on this in the evangelical community but I would be open to a legal category of civil partnership. Gay people could have a specified number of legal rights that would encourage their ongoing commitment. But what really matters, and what’s really decisive, is what marriage means — you may have seen Susan Shell, she’s a liberal, and wrote a piece called “The Liberal Case Against Gay Marriage,” and what she says is what I what I say — that is, the reason every civilization in history has defined marriage between men and women, is that society has a lot at stake in preserving continuity, in a wholesome way. It’s quite clear that when men and women who have sex and make babies stay together. It’s better for their children, and it’s better that children grow up with their moms and dads — and that’s why societies have defined marriage, to protect making babies. The real question is, what is marriage?

(via)