Marriage as an adult right
The Family Research Council, in its opposition to the California Attorney General's support for immediately allowing marriage equality, has stated that the family is the foundation of civilization - to which we all say - that's the point! In a very real sense, to deny gay couples marriage equality is to deny them full participation in civilized society. As such, conservative opposition is more than preserving a cherished institution. It is an assault on the adult rights of gays and lesbians.
In pre-modern civilization, there was no real age of majority. You were an adult when started your own family (back then, if you were gay, that was a side issue - marriage was about acquiring female property). Even with ages of majority, until you marry, you have still not really left your family - as they maintain kinship with you in extremis. In other words, if you cannot speak for yourself and are unmarried, your parents and siblings have the right to speak for you. You are still their child.
When people marry, the right of familial decision passes to the spouse. That is part of being an adult in a very real sense, the ability to chose to not let your family of origin have any say in your affairs and instead pass that responsibility to a person of your own choosing. Without marriage equality, gays and lesbians do not have that right.
Domestic partnership is no substitute. As the FRC stated, marriage is the foundation of society and that foundation should be available to all people. Abolishing civil marriage and calling everyone partners would just be a lame attempt by conservatives at placating the religious. Recognition of spousal rights came way before there were modern religions. Marriage to a large extent predates society.
The desire to deny marriage to gays and lesbians comes from a desire to control their behavior, to make them conform to conservative convention in order to enjoy full adult rights. Denial of marriage is not just about the ceremony - it is about the recognition of adulthood. For full equality in society, denials of marriage equality cannot stand and they are not a fit subject for either legislation or plebiscite.