Other discussions, regarding the way males and females fit physically together, without requiring unhealthy tearing (anal sex) to occur, and the observed idea that the opposite sexes are required for procreation (without scientific intervention), can be brought up. They are obviously not proofs of anything.
The institution of marriage (who ever said that humans were biologically required (or tended) to mate for life?) is a concept fraught with spiritual significance. God makes a commitment to His people, and uses sexual metaphors to describe His relationship with them, and the force of His passion for them. There was always (in recorded history), a notion of sexual ownership in societies. The Bible claims that we are made in God's image. We have passion and jealousy with our lovers, and so does He. God created sex! He is all about pleasure, in it's proper and whole relationship, where there is intimacy. Physical pleasure without a relationship in the real commitment, God-style devotion and jealousy and utmost care and respect of the other person--to the point where both partners are willing to die for the other in the ultimate fashion--as Christ did for us as sinners--is a physical pleasure stripped of its meaning and intimacy.
To make the analogy of relationship to God and His people, it would be God demanding of us worship and adoration, but not committing to us; being arbitrary, capricious, and fickle. He wouldn't be worthy of whatever pleasure He could derive from us, and we would certainly receive no pleasure from being forced into doing it against our will, which would, in the end, bring Him no pleasure at all, for we would not be wanting to do it, and there is no pleasure in that for Him. If there were, He would have much preferred to create robots who gave Him paper praise.
So in order for God--and us--to derive pleasure from the relationship, He must commit to us on the level that He showed us He did in Jesus, and we must likewise commit to Him. It's all or nothing. This is His character. Everyone knows that what you put into something is what you get out, when it comes to relationships. Why go in half-hearted and not ever experience the real joy and pleasure and peace and intimacy of a relationship with God? Can you imagine falling in love with God? What it would be like to come home to God every day, and what the pleasure of intimacy between you two would be like? It would blow your mind! There would be nothing in this world like it! Do you wonder why the psalmists wrote all of those songs, and why David danced in his underwear in the streets, worshipping God? Ecstasy. :)
Now on the question of why male/female is the exclusive permitted pairing, it is based on the same reasoning: God is consistent with His own character; His modus operandi does not change. There is something indescribably precious about new life. To have created someone--alive, breathing, who looks at you and grows to see, and talk to you, and love you back--how precious is that? God gave us the power of procreation so that we could come to understand His attitude towards us. The love a parent has for a child is only a small amount compared to God's love for each of us. And what of His creative power, upon which our bestowed procreative power is modeled? God embodies both male and female characteristics. In Hebrew, God uses His public persona pronoun with a male designation. In the Holiest of Holies, the most private place where the high priest would go once a year, on the day of atonement, the glory of God dwelt in a physical (i.e. "visible to the priest") representation above the ark of the covenant. (The symbol of the committed relationship that existed between Him and His people.) This glory was described by a feminine word, "shekinah," that brought with it the connotation of a woman being wooed.
And now this incredible Being makes His home in our hearts!
To see God for who He is is to see the union of male and female. People have spoken of their lover being "the other half of my soul," or "my better half." In Genesis, God says about Adam, "It is not good for man to be alone," and promptly creates Eve, who is (no surprise) immediately called by Adam, "Bone of my bone, and flesh of my flesh." Aside from the literal story, the metaphorical thrust of the story is that she is just as much part of him as his own body is, and that their union makes them both complete. It is relationship: central, whole, integral. God embodies the metaphysical "fitting together" that makes the whole. As with the creation power of God as powerfully described with a totally unabashed sexual metaphor--"the phallic Word of God, thrusting into the womb of the void," so is the procreative power described by that same metaphor. Why didn't God make us reproduce like frogs? Female lays some eggs somewhere, male sprays some semen in their general direction, and there is no intimacy--not even physical touch, really, required for reproduction. And why can we procreate facing each other, when most of the animal kingdom has the female with her back to the male? Why do we get the added intimacy of face-to-face communication? None of His design is accidental, and all of it is entirely consistent with His character, and His behavior towards us.
Just as two wombs ("receptive, fertile voids") cannot create something, so do two penises ("creative, virile words") not produce anything but wasted seed, clashing with each other.
Biology does not prohibit outside-of-God's-order sex or pleasure. We have the equipment required to feel physical pleasure, and if that is all we seek, we can find it in a myriad of ways--use old shoes, try necrophilia--they won't fight back or complain of a headache.
Science enables procreation outside of the womb, yes. But Science has also produced a whole lot of stuff that has resulted in destruction of our planet, our relationships with each other, and ourselves. Why do people write science fiction about hellish dystopias created by misusing Science? Because it is recognized that not all we *can* do we *should* do. I am not saying in vitro fertilization is bad (though I think it's selfish to spend so much money on yourself when you could adopt a needy child instead), but only that arguing that because Science says we can do it, it doesn't imply that morals concerning the issue go the way of the dodo. And with all of this love of Science and the theory of Evolution, I wonder at our modern-day two-facedness about it. Using it on one hand to justify doing all sorts of things, while on the other, doing all we can to thwart it. I am reminded of Ted Kaczynski's manifesto, in which (of many things) he accused medical technology of possibly doing more harm to the human race than help. If evolution favors the survival of the fittest, and we use medical technology to help the weak survive and pass on their defective genes, then the human race stops evolving to a stronger position biologically, because not only do all (or most) of existing "weak" genes remain in the pool, new ones introduced remain in the pool, as well, and the pool gets full of "crap" and stagnates, actually devolving the population over time.
But you can make a sure bet that no one is going to buy that argument and do away with any part of the study or application of medicine! You can't build a convincing case for anyone that lets a sick child just suffer and die, because evolution is at work! Or a case that would step up and prevent a couple who cannot conceive from having Science help them get pregnant! We push those defective genes that did not support procreation on to the next generation, and so on. To extend this to its extreme, we would end up with a population totally dependent on Science to continue existing. How "strong and evolved" is that? We would be handicapped and would die out if the technology were ever lost.
"So we control our own evolution!" you could say, "And we do gene therapy and design, and eliminate the bad genes altogether!" Well first, this brings up the question of what are "good genes" and now that we're so all about not discriminating against people, how would we do that--and how many gene interactions provide desireable characteristics that come bundled up together with undesirable ones? What if separating them does greater damage? We can't even debug our own, much simpler computer code--how could we possibly reengineer code Somebody Else wrote without any comments or debugging information? And how can we justify messing with real people to run our experiments? We're messing with human life, here! What ramifications would there be if we screwed somebody's life up by our own error? If you would say, "it doesn't matter, it's for the cause of Science," we turn human beings into just machines for testing theories, and the value of a human drops significantly.
Not to mention that if we *were,* somehow, to be able to succeed, we would inevitably create a world like Gattaca's, in which we are valued for our bodies, but we have lost *who* we are. Where "natural-born" children would be always scorned--how much a robbery of joy in a new baby would that be? And how much more would be lost by the countless lovers who so much did not actually want to conceive, for fear of the life the child would have to face? What joy would be removed from sex, then, even sex in a loving, committed relationship? Abortion--more death, how depressing. Sterility--can't use in vitro, also a kind of death-like ring to it. More medications to prevent conception--more havoc wreaked on womens' bodies for the sake of sexual convenience. The "entirely natural" couple has a huge fear and social stigma on them. Ooh, what joy of sex they must face. But we would reap what we sowed. We want to rob ourselves of our humanity, we will be robbed. Either that, or we devolve. Science is not the answer to all of Life's questions.
Wow...lots of impassioned speech, here. :) Basically, I was trying to show why just because we can do it with Science, it doesn't make it right, or the outcome desirable. Why Life now, even with its flaws, is not so bad compared to some of the dystopias we could create for ourselves when we reject God's view on things, and go our own way.
It all boils down to this: God calls our hearts "mansions" and our physical bodies "temples" for Him to dwell in, of innumerable value. Why fill them with trash and keep Him out?