The idea that gay marriage is wrong because gay couples are somehow unnatural is not often stated openly, but this premise influences other arguments and lies behind many peoples negative opinions about homosexuality in general. For most people, heterosexual relationships are the norm, both in society and in nature. Homosexual relationships are thus abnormal and unnatural; therefore, they shouldnt be validated by the state nor recognized as a form of marriage.
Such arguments are superficially effective because they try to harness the power of apparently neutral and objective categories like nature and natural in support of ones position. In this manner a person can try to slough off accusations of bigotry and intolerance because, after all, its just a matter of factual observation as to what is and is not a proper part of the natural order and/or what is mandated by natural law. Its no more bigoted or intolerant than observing the dropped objects fall down rather than up, or that bears mate with other bears rather than with deer.
In reality, however, claims about the natural order or natural law only end up being masks for religious, political, or social prejudices including those that rise to level of bigotry. The philosophical veneer might at times be impressive, but we must not fail to look beneath the surface in order to understand what the real ideas and arguments are. One means for doing that is to ask the not-so-easy question of just what is meant by natural and unnatural.
A common and simplistic meaning is that heterosexual relationships are natural because that is what we find in nature, whereas we dont find homosexual relationships. The latter are therefore unnatural and should not be validated by society. A perfect example of this attitude towards the unnaturalness of homosexuality was expressed by Peter Akinola, Anglican Archbishop of Nigeria:
I cannot think of how a man in his senses would be having a sexual relationship with another man. Even in the world of animals dogs, cows, lions we dont hear of such things.
There are many possible objections to this. First, humans are obviously a part of nature, so if humans have homosexual relationships, is that not therefore a part of nature? Second, we dont find dogs, cows, and lions entering into legal marriage contracts with one another does that mean that legal marriage as an institution is unnatural and should be eliminated?
Those objections point to the logical flaws in the argument, revealing what was described above: its merely a philosophical veneer applied to personal prejudices. Just as important, however, is that the argument is factually false. Homosexual activity and homosexual relationships can be found throughout nature in dogs, cows, lions and more. With some species, homosexual activity is quite common and regular. This means that the argument isnt just a philosophical veneer, its a cheap and poorly applied veneer to boot.
Sometimes the argument that homosexual relationships and homosexuality are unnatural might be meant in the sense that it doesnt really flow from human nature in its raw state, untainted by civilization. Presumably this is supposed to mean that if it werent for the society around us, no one would be gay wed only ever want to mate with or have intimate relationships with members of the opposite sex.
There is no evidence offered to back this up not even false evidence, as with the prior argument. Yet even if we accept that it is true, so what? The mere fact that humans wouldnt do something when in a state of nature outside the confines of civilization is absolutely no reason to conclude that they also shouldnt do it when living within civilization. We wouldnt drive cars or use computers outside of the structures of civilizations, so should we stop doing them while a part of society?
Very often the argument that homosexual relationships are unnatural is meant to describe the fact that they do not and cannot lead to the creation of children, which is supposed to be the natural consequence of such intimate relationships, especially marriage. This argument also isnt effective, but the relationship between marriage and raising children is addressed in more detail elsewhere.
Ultimately, the homosexuality is unnatural argument fails to support the case against same-sex marriage because there is no clear and convincing content to the concept of unnatural in the first place. Everything that is claimed to be unnatural is either arguably very natural, arguably irrelevant to what the laws should be, or is simply immaterial to what should be treated as moral and immoral. It's no coincidence that what is "unnatural" also happens to be condemned by the speaker's religious or cultural traditions. Just because some trait or activity isnt the norm among humans doesnt make it unnatural and therefore wrong.