As I promised in the post immediately below this one, I'm going to try to lay out what I think are the secular reasons to oppose gay marriage. Please bear in mind two things: one, I'm not the best person to do this, so I'd appreciate any insight you might have, and two, just because I think focusing on secular reasons would be a good strategy at the present time does not mean that religious concerns are somehow invalid. Not at all, actually. But in our modern world, 'religion' tends to be viewed as something some people choose to do, for what probably seem to be good reasons to them, but not something that in any way belongs in the public sphere. Though I deeply disagree with that notion, that's a conversation for another day.
Why oppose gay marriage, then? If marriage is all about two people loving each other, what difference can it possibly make whether the people are the same or different genders?
The first thing we have to do is to point out that, in the context of the law and the eyes of the State, marriage has nothing whatsoever to do with love. There is no checkbox on a marriage license which asks the question, Do the applicants solemnly swear/attest that they love each other? The law doesn't really care.
In fact, the law is largely indifferent to most human relationships. The law doesn't ask how often you visit your grandmother, or whether you call your sister. The law doesn't spell out whether second or third cousins must be invited to family reunions, or how much affection you owe your aunts or uncles. The law refuses to define friendships, doesn't care if you're dating casually or seriously, yawns at the news of your engagement (unless you've signed a pre-nup, in which case the law files that away for future reference). But the law abandons its default position of non-interference in human relationships when it comes to marriage.
Why? What gives the law the right, or the State the compelling interest, to get involved in marriages when it doesn't generally get involved in relationships except peripherally?
Marriage is a legal contract. The contracting parties are promising each other certain things; there are mutual rights and responsibilities involved. The law takes such legal promises rather seriously. This does not mean that the law expects every individual who contracts marriage to be able to live according to these promises, but it's very important to note that the law isn't generally concerned with individuals. The saying "Hard cases make bad laws" comes from this idea; the more the law is written with specific people in mind, the worse the law ends up being for the general public.
So what are the legal promises of marriage?
First: the intention of permanence. It must be noted that the law is a cynic, not a romantic; it stands by with all sorts of laws pertaining to divorce. But there's no such thing as a temporary marriage license. When a couple gets married, they must at least maintain the legal fiction that they intend it to last.
Second: the intention of exclusivity. The law is fairly liberal on this one; it doesn't really care if Mr. Brown marries Miss Grey with the understanding that he will continue to see Miss Green. The general policy of noninterference in human relationships is pretty strong, here. But most states will still allow Mr. Brown's dalliances with Miss Green to be used in a divorce action; more importantly, if Mr. Brown attempts to marry Miss Green while still legally married to the former Miss Grey the law will have quite a lot to say to him. Bigamy is illegal precisely because it violates the marriage contract's promise of exclusivity.
Third: the promise the couple makes to each other to share their material goods, not only with each other, but also with and for the benefit of any children who are the natural and expected result of the relationship.
I've emphasized that last, because it's the real reason the law gets involved in marriages at all.
The law, looking at human history, and the mystery of human biology, draws certain conclusions. It concludes, not altogether unreasonably, that heterosexual relationships are extremely likely to produce children, and that if certain safeguards aren't put into place, it is entirely possible that those children will end up on the law's, or rather, the State's, hands. Again, the law is a cynic. While its human officers may expect such things as natural affection and a natural desire on the part of the parents to care for and protect their offspring, the law itself takes no such rosy view of human nature, and concludes dispassionately that it is within the realm of possibility for a father to abandon his children to poverty or a mother to neglect their well being if the law is unwilling to demand the good of the children as a condition of the marriage. In fact, viewed properly, the intentions of the couple as regards permanence and exclusivity exist not for the sake of the couple themselves, but for their putative offspring.
Now, as I said above, the law isn't concerned with individuals. Some individuals may be infertile; some may be too old to have children; some, in a secular world, may choose never to have children (though that last choice can't really be considered a permanent decision until the couple reaches an age at which conception is no longer a physical possibility). The law understands this, but it is willing to overlook the fact for the sake of the vast majority of marriages which do produce children, and for whom all laws pertaining to marriage are actually written. The tax laws which favor marriage exist to promote family stability, a good thing for the children. The inheritance laws which take husbands or wives and children into account also exist primarily for the children. In fact, any law which seems to favor married couples over unmarried ones, or single people, does so in part to offset what the law sees as the onerous burden of raising children, a burden the law doesn't want to have to shoulder in the family's absence.
So far-reaching is the law's interest in children that the law takes the unusual step of sometimes interfering in relationships which are not marriages for the sake of children produced by these relationships. A woman who regularly cohabitates without marriage may find that the law is more than willing to interfere, on behalf of any children she might have, if she moves in with a man who is a registered sex offender, for instance. Her freedom to define herself and choose her own relationships is not absolute, and the law has no qualms about making her choose between her children and her newest male partner.
So how does all of this relate to gay marriage?
Simple. Gay relationships are fundamentally dissimilar to heterosexual ones precisely because gay relationships can never of their own initiative produce offspring who are the biological and natural responsibility solely of the same-sex couple.
A lesbian couple, for instance, who uses donor sperm to conceive has produced a child who is biologically related to only one of them, and who must be adopted by the other. A third party, the sperm donor, is the actual father of the child no matter what we pretend about this group of people for the sake of political correctness.
A gay male couple can possibly donate sperm to be used by a woman to produce a child for them. However, the woman is still the child's mother, again despite our pretenses. And absent adoption, the child will only be the natural responsibility of whichever man is the child's biological father.
A gay couple of either gender can also adopt in most states; but here the law sees no need for marriage, since the law's willingness to interfere in adoption doesn't depend in the least on the marital status of the people who wish to adopt.
In all three of these instances, the law recognizes the fact that these relationships are not analogous to marriage, precisely because the children are not the natural and expected result of the relationship. A gay relationship can never produce children on its own. A gay couple, as a couple, will never find themselves unexpectedly pregnant and unprepared for what the law sees as the onerous burden of raising children, and will therefore not need the law's succor to help them shoulder this burden. In fact, the gay couple as a couple only takes on the burden of raising children by bypassing the natural means of becoming parents and either creating a child with the help of a third party (the child's natural mother or father) or by adoption, or both.
And the law is well aware that, unlike the handful of elderly or infertile heterosexual couples, all gay couples by definition are incapable of becoming the biological parents of the same offspring. Since the law's interest in regulating marriage is overwhelmingly due to its concern for the children who are the natural and expected result of marriage, why should the law change at all?