There are many rational reasons to oppose gay marriage. The first is that gay sexual relationships, particularly those involving men, result in far more STDs than those involving straight couples. The gay community has a much higher risk of acquiring them than other communities. Further, gay couples give no benefit to society - they cannot procreate, a major reason for the current definition of marriage. Finally, significant portions of the population object to having gay marriage forced down their throats.
A reasonable compromise is civil unions. Civil unions provide the same legal benefits and obligations that marriage does - satisfying the main complain of the gay lobby - but recognizing that they are different than marriage - satisfying those in opposition to gay marriage. Arguing that civil unions are not the same as marriage is simply arguing over semantics. Neither side will be completely happy with it, but that is what happens when you compromise. In this case, it gives both sides the core of what they want.
Maybe not good ones--or even true ones, for that matter--but there most definitely are secular reasons to ban gay marriage. "It'll ruin the economy!" "It'll be bad for our children!" "Think of the kittens!" All of these are secular arguments against gay marriage. Not a single one of them makes sense.
Those that deny there are any secular arguments are emotive, thoughtless bigots !
(One doesn't have to believe that the secular arguments are persuasive.)
Why did marriage emerge tens of thousands of years ago - before organised religion - and within distant, separate cultures?
Sometimes it was about power, authority and land - but mostly it was about a framework for protecting children. (Of course not every marriage produced children....)
Before the bible came along, homosexual liaisons may have been tolerated - in some societies possibly even "normal" - but, for millenia there was no reason to extend marriage to homosexuals.
The only reason seems to be "political", an excuse to attack organised religion (which certainly needs to be challenged on many issues).
So in short, the best framework for raising children is a mother and father, preferably (but not necessarily) in a CIVIL marriage (to remind
the parents of their ongoing responsibility).
A necessary (though not sufficient) condition for marriage is the possibility of the marital act, i.e. sexual intercourse. Two individuals of the same sex cannot engage in sexual intercourse self-evidently. Hence, a necessary condition for marriage is lacking to all gay marriages. Thus, gay marriage is a impossible given our genetic makeup, just as square circles are impossible given the postulates of Euclidean geometry. This is a secular reason to "ban" gay marriage, although the word "ban" is a little inappropriate in this situation.
10. It is unnatural. Americans always ban unnatural things like eyeglasses, polyester, and air conditioning. (And actually, no, it isn't unnatural. It occurs in nature, among species which are not human. You may think being gay is a choice - when did you choose to be straight? And even if it was a choice, so what? People choose to be assholes, and they can still get married.)
9. Gay marriage will encourage people to be gay, in the same way that hanging around tall people will make you tall.
8. Legalizing gay marriage will open the door to all kinds of crazy behavior. People may even wish to marry their pets because a dog has legal standing and can sign a marriage contract.
7. Straight marriage has been around a long time and hasn't changed at all; women are still property, blacks still can't marry whites, and divorce is still illegal.
6. Straight marriage will be less meaningful if gay marriage were allowed; the sanctity of Brittany Spears' 55-hour just-for-fun marriage would be destroyed.
5. Straight marriages are valid because they produce children. Gay couples, infertile couples, and old people shouldn't be allowed to marry because our orphanages aren't full yet, and the world needs more children.
4. Obviously gay couples will raise gay children, since straight couples always raise straight children.
3. Gay marriage is not supported by God. In a theocracy like ours, the values of one religion are imposed on all. That's why we have only one religion in America.
2. Children can never succeed without both a male and female role model in the home. That's why we, as a society, expressly forbid single parents to raise children.
1. Gay marriage will change the foundation of society; we could never adapt to new social norms. Just like we haven't adapted to cars, the service-sector economy, or longer life spans.
The only secular argument I've ever heard against it was that the point of a species is to reproduce, and marriage is nothing but a better way to raise your offspring. However banning homosexuals from being married isn't magically going to make them straight and reproduce, but if they are married they can raise children as best as they can and help society as much as they can.
No, there are no secular reasons to ban gay marriage. Every reason given for banning gay marriage is of religious motivation. In fact, there are many secular reasons for allowing gay marriage, such as recognizing the validity of the love between those two individuals. Thus, there are no secular reasons for banning gay marriage.
It certainly holds no validity anyway. I've heard some faux-secular reasons usually rooted from religious research. In fact, most anti-gay rights research I've encountered has been funded in part or in whole by some form of religious group or group representing a radical anti-gay stance already. Such secular reasons include but are not limited to the "fact" that homosexuals are child molesters. This is clearly untrue and needs to be debunked in everyone's mind immediately. Actual valid research stands to oppose such a sentiment.
The main secular reasons I hear are that it's not natural and that it will ruin family life. People who say being gay isn't natural don't understand two things: 1. Homosexuality appears in almost every species of animal and 2. Marriage isn't natural.
As for ruining family life, there is no evidence to back that up. In fact, I don't really know what that means. My guess is that they think a child needs a mother and a father, which has no evidence to back it up.
No, there do not seem to be any obvious secular reasons to not change the definition of marriage to include gay relationships.
However, the freedom of religion means that Americans are free to take religious convictions into account when deciding what their stance will be on any given issue, if they so choose. Separation of church and state means separation of church and state, not separation of religious or moral convictions and state.