The moral objections of one group should not restrict the freedoms of another. Gays and lesbians have long been condemned for their alleged promiscuity, but when they ask for the right to establish socially and legally recognized, monogamous relationships, their commitment is judged to be a threat to society. Such rhetoric is simply hypocritical. Proponents of the act charge that marriage is an institution already in trouble, and that expanding it to include gays would further weaken it. Marriage doesn't need to be defended from people who want to commit themselves to lifelong relationships.
Marriage is more than an emotional commitment, however.
A woman can marry a man after knowing him only a short time and instantly gain a host of legal and economic benefits and protections. If that same woman has a committed long-term relationship with another woman, the situation changes dramatically. If her partner falls ill or dies, she has no more legal authority than would a neighbor or roommate. Not too long ago, our society prohibited interracial marriages on similar grounds. History has proven such claims erroneous.