They have sex for mutual satisfaction on many levels, but not for the procreative purpose which is, by God's design, its structural, psychological and spiritual core.
They can't explain why sex should just be between a man and a woman, because if fertility isn't a factor, the different anatomies have no significance. "Gender" (as they call it) has no significance.
People who have deliberately ditched this level of significance (contraceptors and sodomists) will eventually be having sex with warm, responsive, rechargeable Japanese androids, and "person" will have no significance.
The current issue (Aug/Sept) of First Things has an article that I think is relevant: "Technocracy Now" by James Kalb. Mr. Kalb says that the "project" of the political and cultural elite that has brought us "gay marriage" can be described as "maximizing the satisfaction of preferences." This view of the "good" for which we strive permeates nearly all of modern life, including "conservatism."
If "heterosexual marriage" is simply an institutional vehicle for "maximizing preference satisfaction" for the spouses, then how does it differ, in essence, from "gay marriage"? It differs only in the accident of the presently-preferred sexual activities of the participants.
In my opinion, without a much grander picture of holy matrimony, conforming to both the natural and spiritual realities of the human person, no ground in the public space can be gained. This doesn't mean that individual couples won't live out holy matrimony, although that's harder when "preference satisfaction" is their criterion whether or not they're aware of it. However, the general view of marriage has nothing new to offer people who see the goal of human existence as maximizing preference satisfaction with a minimum of constraint.
(Pardon all the "quotes": I'm working with someone else's idea which I have not fully absorbed, although I feel as if I've been on the verge of coming up with something like this concept.)
....”sodomitic acts are a turning away from natural sex, and these acts are sterile”...
No...I think that The scriptures are clear these are people who have turned away from God....as follows:
“For although ‘they knew God’, they neither glorified him as God nor gave thanks to him, but ‘their thinking’ became futile and their foolish ‘hearts were darkened’..... .....
...’Therefore’.... God gave them over ‘in the sinful DESIRES of their hearts ‘to sexual impurity for the degrading of their bodies with one another’.... They exchanged the truth about God for a lie, and worshiped and served created things rather than the Creator—who is forever praised. Amen.
BECAUSE OF THIS... God gave them over to shameful lusts. Even their women exchanged natural sexual relations for unnatural ones. In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed shameful acts with other men, and received in themselves the due penalty for their error.
Furthermore,... just as they did not think it worthwhile to retain the knowledge of God, so God gave them over ‘to a depraved mind’, so that they do what ought not to be done.
They have become filled with every kind of wickedness, evil, greed and depravity. They are full of envy, murder, strife, deceit and malice. They are gossips, slanderers, God-haters, insolent, arrogant and boastful; they invent ways of doing evil; they disobey their parents; they have no understanding, no fidelity, no love, no mercy..
... Although they know God’s righteous decree that those who do such things deserve death, they not only continue to do these very things but also approve of those who practice them.... Romans 1:21-31
The primary purpose of sex between a husband and wife is summed up by Christ(interesting he did not mention children in this context) Mark10:But from the beginning of creation, God MADE THEM MALE AND FEMALE. 7”FOR THIS REASON A MAN SHALL LEAVE HIS FATHER AND MOTHER, 8AND THE TWO SHALL BECOME ONE FLESH; so they are no longer two, but one flesh.…”
In this context he was speaking of divorce but he in turn was also quoting Genesis 2:23-24:…23The man said, “This is now bone of my bones, And flesh of my flesh; She shall be called Woman, Because she was taken out of Man.” 24For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh.
In this context the Bible was speaking of the creation of a woman out of man via the rib of Adam and that both man a woman find a completeness thru one another. Children were not spoken of in this context.
God did command that we be fruitful and multiply but the primary purpose of sex is to strengthen and reinforce the soul and spiritual ties between husband and wife.(to become one flesh...which is why God really hates divorce for it injures the souls of 2 people and children if present) That a couple however, should hope to continually remain childless may indicate a darker spiritual issue as you have stated but not for those couples who desire to have children yet God has chosen NOT to bless them as parents of such!