Once we believe (in creed or practice) that humans are only biological, self-gratifying, need driven creatures, the next step is to apply this logic sexuality. Sex becomes only and essentially a means to gratify our desire (or need, as Maslow might say). The same worldview that drives our mass-consumer world ends up driving our sexual ethics as well. After all, “sex sells.”
We’ve become accustomed to throwing off “old-fashioned,” especially religious, moral principles. The only moral principles now are do what makes you happy and don’t cause suffering in others. Thankfully humanism has maintained the latter, but even with it, this moral system is incomplete and contradictory at best.
In this moral system, where there are relatively few constraints, there is nothing obviously wrong with promiscuity, pornography, polygamy, incest (so long as no children are produced), homosexual behavior, prostitution, etc. If sex is only a biological act then our only course is to simply follow our biology. Or, to quote the lines of a song popular when I was in school, “you and me baby ‘aint nothing but mammals…” (I’ll refrain from finishing out the lyrics.)
Is there another way to view human sexuality? I think a proper understanding of Creation gives us a more complete view. We see, first, that people are both physical and spiritual beings. All our physical acts are also spiritual acts. This is especially true in the area of sexuality. God created us male and female. Marriage, and the act of consummation, is a means by which husband and wife are united as “one flesh.” God Himself has bound husband and wife together (Matthew 19:4-6). The apostle Paul later explains how this marriage union is a picture of Christ’s union with the Church (Ephesians 4:25-32). Again, Christians are warned against becoming “united” with a prostitute. After all, Christians are already “united” with Christ (1 Corinthians 6:12-17). Notice how spiritual and physical realities are intertwined?
God created the institution of marriage as a covenant relationship and sex as a means of bearing children and expressing covenant love within that context. That covenant is not just physical (though it is physical) but spiritual as well. That covenant, and the expression of it is sacred. It is holy. Any violation or misuse of it is a violation against or misuse of something which God has set apart as holy. But, if we recognize God’s beautiful design, then marriage and sexuality are far more meaningful and gratifying than anything the world has to offer in its cheap substitutes.
I could end here but I want to caution against one more error. There are some who have gone the other direction and have divorced sexuality from biology all together. In this view, what happens physically doesn’t matter at all – it’s only about expressing love. In this view no physical acts are inherently wrong because what happens in the physical world doesn’t really matter. This is an old, old view called Gnosticism but it has been trumpeted once again to promote a permissive attitude toward any kind of sexual behavior. This is essentially the view of John McNeill. Again, by God’s design we are both physical and spiritual beings. To deny either of these truths is to fall into error.
 Even this shows signs of going away. If you really want to feel depressed, check out Peter Singer’s views on infanticide. Humanism ‘aint so great, but it sure beats his brand of Utilitarianism.
 I have heard it argued (I believe it was Lesslie Newbigin) that our society already functions with a sort “serial polygamy.” Multiple wives (or husbands) isn’t really abnormal in our society, we just don’t have them all at the same time.