Here is an excellent essay on why some people are threatened by gay marriage. Not because gays marrying threaten the institution of marriage, but because they threaten traditional gender roles.
I’ll include one quote, not because this captures the entire (relatively short) essay, but just to get you to go read it.
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. Over and over on C-SPAN I hear people in 2013 arguing that both a mother and a father are needed in order to raise children – indeed, that children have a RIGHT to both a mother and a father. (And so, you see, proponents of same-sex marriage are not actually supporting the granting of rights, but rather the taking away of rights… of children. The twists in logic are mind-boggling.)