I've noticed that Christians who resort to the design argument seldom seem to understand the full implications of their position. Focused as they are on the finger they're busy pointing at the "unnatural" actions of gays and lesbians, they're completely blind to the fact that the rest of their fingers are pointing back at them.
God's design, as revealed in Genesis 1-2, cuts both ways.
On the most superficial level, the "anatomy" argument states the obvious fact that male and female sexual organs were designed to fit together, and concludes from this observation that a man and a woman are required in order to use them properly. The logical implication of this conclusion, however, is that all other forms of sexual activity (oral, anal, mutual masturbation and whatever else is possible) are sin, even if done by a married heterosexual couple. The parts just weren't designed to fit together that way, after all.
Even more serious than that, however, is the issue of divorce, which attempts to sunder the sacred bond of marriage. Divorce is allowed by the Bible under certain circumstances (infidelity, abandonment and arguably certain forms of abuse), but remarriage by a divorced individual is unconditionally condemned by no less an authority than Jesus himself.
Anyone who divorces his wife and marries another woman commits adultery, and the man who marries a divorced woman commits adultery. (Luke 16:18, NIV)
That alone should be enough to settle the matter. Remarriage, after all, violates God's original design (one man plus one woman for life) by introducing a third person into the mix. The first spouse may not be physically present, but as the Apostle Paul points out, the spiritual bond formed through sexual union is broken only by death.
Nevertheless, few churches today treat remarriage as anything worse than two people making the best of an unfortunate situation. You never see Christian activists lobbying for bans or even restrictions on second (or third, fourth, fifth, etc.) marriages; for the most part, they've even largely given up on rolling back "no fault" divorce laws.
And the reason is obvious: remarried couples make up a significant percentage of the average congregation; ostracizing them would empty out the pews of many churches, large and small. Gays, on the other hand, are a small enough minority that their absence is barely noticed at all, and thus the blatant hypocrisy of condemning one group of 'sinners' but not the other becomes a matter of expediency.
So what, if any, justification is there for condoning remarriage following divorce in the face of Jesus' own words against it? Just one: the active presence of the Holy Spirit in the lives of many remarried Christians. Having seen firsthand how God works through and blesses the second marriages of individuals that the letter of the law calls 'adulterers,' I cannot state with any certainty that a divorced person's only legitimate options are reconciliation or lifelong celibacy. Some second marriages probably are blatantly adulterous, but I, for one, am not wise enough to determine where the line should be drawn.
In light of this, what are we to do with gay Christians whose lives and relationships demonstrate that the Holy Spirit is actively at work within them? Even if 1 Cor. 6:9 is referring to all homosexuals (which is far from certain), the fact that some 'adulterers' apparently can inherit the kingdom of God suggests that we may have misinterpreted Paul's intent in that passage.
The letter of the law leads to death. What does it really look like to choose life?