Compelling State Interest
Perhaps if we look at why marriage laws came about in the first place, we would have a better idea of who would qualify. What is the state interest, compelling or otherwise, to promote marriage through the incentives it provides? I would suggest these:
1. To promote a stable, lifelong, monogamous sexual relation between two people.
2. To provide protections for each member of the couple and their children.
A monogamous relationship ameliorates the ill effects both physical (STDs) and emotional (alienation, jealousy, instability) that infidelity can bring.
Protections include the financial and emotional support for the partner, if needed, but especially for the children.
Since marriage promotes monogamous sex, it can never be for more than two people. Since it involves sex, it cannot be for mere convenience, like two maiden aunts marrying for the legal privileges marriage provides. This doesn't cover incest, but that is prohibited for other compelling public interest reasons. Marriage is a legal contract, so the suggestion that people in this "anything goes" culture will soon be allowed to marry their pets is ludicrous .
So who can marry? Promoting monogamy between homosexual couples is as important as promoting it between heterosexual couples, perhaps more so. Those who complain about homosexual promiscuity should fully support gays and lesbians who are willing to commit to a monogamous relationship through marriage.
Four million children in this country are being raised by at least one gay or lesbian parent. Most are the result of previous heterosexual marriages that ended in divorce. Allowing a single gay or lesbian parent to marry his or her same sex partner would improve the emotional stability and financial security of these children just as the remarriage of a heterosexual divorcee does. I'm talking in generalities, of course. Don't come back with stories of ugly step-parents.
The children of gay and lesbian couples who become parents through adoption or artificial means deserve the same privileges and protections that marriage provides the children of married heterosexual couples. Treating children differently based on the sexual orientation of their parents seems like blatant unjustifiable discrimination to me.
If encouraging monogamous relationships and providing support and protection for children is the object of our marriage laws, then SSM should not just be allowed, but vigorously promoted.