This is a big topic in California these days, and yes, it's another of those touchy subjects. Despite the knowledge that I will offend someone somewhere, I think I have to take a stand.
The basic economic unit of our civilization is the household, and the first defining feature of a household is the relationship of the people who reside together. Unmarried people have the benefit of pooled resources; married people also get tax, insurance, and inheritance benefits. Marriage, then, is basically an economic decision. We get married so we can improve our financial security and the security of our children.
The question of sex is secondary, and - all fantasy aside - is settled in a variety of ways depending on age, attraction, and compatibility. There is an expectation that marriage will entail a sexual relationship, particularly in the beginning, but in practice the sexual relationship may or may not endure. Every marriage, if it lasts long enough, will see the sexual aspects diminish in importance; the economics never do.
In general, who has sex with whom has been the province of religious institutions, guided by tradition. The state has no real stake in this except where sexual behavior may affect the physical or mental well-being of the community, as in incest, bestiality, or sexual violence. Clearly, where public safety is an issue, the state can and should set prohibitions. Public safety is not threatened, however, by consenting adults engaging in intimate relationships, whether they are heterosexual or homosexual.
I believe that states and governments should preside over civil unions alone, and that these can be defined as between any pair (or even group) of people who apply. Marriage, with its sexual dimensions, should be the province of churches, synagogues, and temples.
That's my story, and I'm sticking to it.