Marriage is an institution for raising children.
Before you say that you believe this, answer a few questions:
- Should childless post-menopausal women be bared form getting married?
- Should a childless woman be given an automatic divorce if she has a hysterectomy?
- Should couples who have not had children be given an automatic divorce after a reasonable period?
Unless you agree with these, you don't really believe that marriage is only for raising children.
"What about adoption?" you may ask. What about it? Gays can and do adopt and studies show that it is better to have two gay parents than none.
Marriage is a religious institution and the Bible forbids it.
Marriage straddles the line between being a civil and religious institution. Historically, Catholics saw it as religious and puritans saw it as civil. That is why both a clergy and a judge can perform a marriage. In this case we are talking about the civil portion of it.
There is also the First Amendment which forbids passing laws based on religion.
Same sex marriage is an attack on the institution of marriage.
Marriage is in a lot of trouble but not because of a tiny minority. An increasing number of hetro couples skip getting married and an increasing number of women have children by multiple fathers before marrying (and seldom to the father of her children). A distressing number of presidential candidates (and one ex-president) have been caught in affairs. Having a new group that wants to be married can only strengthen it.
Same sex marriage will lead to polygamy.
Maybe it will, maybe it won't. That's a different fight for a different day. There are a lot more people in same-sex relationships than multiple-partner ones. But those relationships do exist and they are pushing for recognition regardless of same sex marriage. The two are not directly linked.