From this blog post:

…same-sex marriage threatens traditional marriage because it challenges ideas about proper gender roles.

Same-sex marriage makes a lie of the very foundation of traditional gender roles. Same-sex marriages say that a woman can run a household, or that a man can raise a child. This does not square with those whose lives and beliefs and relationships depend on upholding and living their lives based on differences between the sexes.
….
I am struck in listening to the opposition to same-sex marriage by the persistent denial that gender is a socially constructed role. This is a “traditional” view of marriage in the sense that it is grounded in “biology is destiny,” or specific roles assigned based on sex. It is an extremely narrow view of “marriage” based on specific roles assigned by sex, rather than marriage as an emotional and physical and social partnership between two individuals. Most telling, it is a view that denies that heterosexual people can be in egalitarian marriages, or should be. It is a belief in “traditional” marriage as hierarchical. Not as a true partnership of equals, but as a microcosm of society with a power structure that flows from husband to wife to children.

 

Interesting read.