You're simply choosing an arbitrary definition of marriage which cannot be supported if you think that what we traditionally view as marriage and what is grounded in Natural Law (man and a woman)
You have two options for "Natural Law"
(1) The "Natural Law" seen by humans, and interpreted for some meaning. (philosophers and theologians, ie Aristotle)
(2) The "Natural Law" shown by nature without any further interpretation needed (scientists and naturalists..ie Descartes, Bacon, and others of the scientific revolution)
The first, which asks "why," will inadvertently move away from what is actually natural and can eventually collapse under inquiry or historical experience. The second, which asks "how," doesn't.
Bees and primates, for eample, engage in homophobic activities. Bees and primates are part of nature and therefore it is natural. End of story. Thats it. Homosexuality, is a rare, but natural occurance of the biosphere. The only reason to detest it is if one follows the first view of "natural law" which always seem to blindly extrapolate laws of nature.
Religion aims for a certain perfection without realizing that there can be perfection in seeming chaos. Exceptions and rarities do not disprove laws; it reinforces them!
I'll try to present Natural Law as best as possible. This is of my own free will, so I'm not terribly inclined to delve into a comment, defend, comment, defend type situation. Due to the inherent complexity of explaining such a philosophical concept, this post will get long. Sorry. And don't believe that will write this without the help of literary companions, etc. I am not a professional philosopher of ethics and natural law.
Natural Law theory is very badly understood by those who criticize it. Common objections go something like:
"If it's wrong to go against nature, then isn't it wrong to wear glasses or drive cars since these are artificial?"
or
"If what's good is what is natural, isn't everything good, then, because everything that happens in nature is therefore 'natural'?"
or
"If homosexuality is genetic, doesn't that show that it is natural too?"
etc.
Aristotle takes a thing's form, essence, or nature to determine the good for it. Hence a "good" triangle is one that corresponds as closely to the form of triangularity as possible. Accordingly, a "good" squirrel is one that has the typical marks of the species and one that successfully fulfills the the characteristic activities of a squirrel's life, etc.
When we turn to humans, we find that we too have a nature or essence, and the good for them, like the good for anything else, is defined in terms of this nature or essence. Unlike other animals, though, humans have intellect and will, and this is where moral goodness enters the picture. Human beings can know what is good for them, and choose whether to pursue that good. And this is the natural end of the faculties of intellect and will; for like our other faculties, they too have a final cause, namely, to allow us to understand the truth about things, including what is good for us given our nature and essence and to act in light of it. Just as a "good" triangle is a triangle that most closely approximates the form of triangularity, so too a good human being is one who successfully carries out the characteristic human life, as determined by the final causes or natural ends of the various faculties that are our by virtue of our nature or essence.
The will of its very nature is oriented to pursuing what the intellect regards as good. You don't need to believe in Aristotelian final causes to see this. You know it from your own experience insofar as you only ever do something desirable or providing some benefit. Human action is of its nature directed toward what is perceived as good in some way, whether it is truly good or not.
Suppose then (if you must, in the name of "for the sake of the argument"), that things really do have final causes, including our various biological capacities. Then, obviously, the final cause or natural purpose of sex is procreation. And procreation is inherently heterosexual (the fact that people can be cloned or that some people have sex other than for procreation like pleasure is irrelevant; it isn't important what
our purposes are, rather it is important what
nature's purposes are in the Aristotelian sense of final causality). In human beings, procreation is not just a matter of producing new organisms, but also forming them into persons capable of fulfilling their nature as distinctively rational animals. The final causality of sex thus pushes inevitably in the direction of at least some variation on the institution of marriage, and marriage exists for the purpose of not only to generate and nourish offspring biologically but also culturally.
And, now, for the more awkward part of discussing how Natural Law deals with the sexual act...
If we consider the structure of the sexual organs and the sexual act as a process which, through the prospect of pleasure, begins with arousal and ends in orgasm, it is clear its biological function, its final cause, is to get, uhm, semen into the vagina. The organs fit together like lock and key. This all blindingly obvious, though, and you'd be unreasonable to deny it. But from the point of view of biological final causes, all of this exists only so that men and women will engage in a sexual act, so that it will result in the deposition of sperm into the vagina, so that in turn offspring will be generated and so that the father and mother will be strengthened in their desire to stay together which is nature's way of sustaining that union upon which children depend for their material and spiritual well-being.
If there really are Aristotelian natures, essences, final causes, etc, then the lesson of all this for sexual morality should be reasonably obvious. Since the final cause of human sexual capacities is procreation, what is good or human beings in the use of those capacities is to use them only in a way consistent with this final cause or purpose. This is a necessary truth -- for the good for us is defined by our nature and the final causes of its various elements. It cannot possibly be good for us to use them in any other way, whether an individual thinks it is or not any more than it can possibly be good for an alcoholic to indulge his taste for excessive drink. This remains true regardless of the reason for someone's desire to act in a way contrary to nature's purposes (no, this doesn't mean that people must always intend to have children with every sexual act nor does this mean that all organs must serve only one core function).
Natural Law doesn't condemn using a natural capacity or organ other than for its natural function, but only in a manner contrary to its natural function, frustrating its natural end. Natural Law also does not entail that every frustration of nature's purposes is a serious immorality. Where certain natural functions concern only some minor aspect of human life, a frustration of nature's purposes might be at worst a minor lapse in virtue like prudence. But when they concern the maintenance of the species itself, and the material and spiritual well-being of humans (as is the case with sex), acting contrary to them cannot fail to be of serious moral weight.
Cute, Omega, but how does this have to do with homosexuality and "same-sex marriage"?Does natural law entail that homosexuals cannot marry? They
can marry. But of course, what that means, as a matter of conceptual necessity is that they can marry someone of the opposite sex. What they cannot do is marry each other, no more than a heterosexual could marry someone of the same sex, and no more than a person could "marry" a fish or a can of motor oil or his own right foot. For the metaphysics underlying natural law theory entails that marriage is, not by human definition, but as an objective metaphysical fact determined by its final causes, inherently procreative and thus inherently heterosexual. There is no such thing as "same-sex marriage" any more than there are round squares. There is even no such thing as "sex" outside the context of sexual intercourse between a man and a woman. Sodomy, between two men or a man a woman or two women, no more counts as "sex" than puking up some pizza counts as eating. No ordinance, legislature or opinion could possibly change these facts any more than they could repeal the law of gravity or the Pythagorean theorem. Any "law" that attempted such an impossibility would be absolutely null and void, a joke at best and a straightforward assault on the very foundations of morality at the worst. For if "same-sex marriage" is
not contrary to nature, then nothing is and if nothing is contrary to nature, then there can be no grounds whatsoever for moral judgment.