The nature of ‘the act’ of sexual expression between a husband and wife is that they become one flesh - the first and primary end of their relationship is companionship and unity.
Does the part ofter the dash explain the part before the dash?
Secondarily, the end of sexual expression in marriage may be for procreation, but that is not the radical nature of the sex act; it is the sexual expression (and sex is only one component of the contract and covenant of marriage) of a relationship of mutuality and care.
You got a source for this? Despite the great admiration for making whoopee in the Song of Songs, I would imagine those folks knew the usual relationship between "baby carriage" and "love and marriage", and I'm saying that the sexual act in all its fullness would include procreation just as the chewing and swallowing act in all their fullness (so to speak) include nutrition.
I think there is a gnostic tinge maybe (this is not an argument, not at all, it's kind of a "here's where I may try to go with this" statement) to separating the sexual act from its social, economic, and biological side and making it "radically" about the relationship between husband and wife simpliciter. In fact I think that a lot of the pressure to "perform" (perceived and real) comes from that divorce.
Again, it's no kind of argument, but ABC was touted as the great hope for marital sexual frustration and for illegitimacy, remember? It SURE didn't work out that way.
Principle for consideration: Just because Chimpanzees do something doesn't mean it's not perverse. Goats masturbate. The occasional ram (I ran sheep and goats for a while - goats are cooler - but stinkier) will attempt to mount another ram. (If all he mounts are other rams he ends up in the pot.) And certainly ewes will overeat - with disastrous consequences. So the argument from "what critters do" to "what an act 'is'" is not a slam-dunk.
I would question the idea of sinful (perverse) chimpanzees. I simply raised the issue of natural example, because that is one of the bases for the Aristotelian tradition’s argument for what is ‘natural’ vs. ‘unnatural’ sex.
I provided Biblical references for my viewpoint in a previous post. As a former Anglican priest, I am sure you are well aware of the tradition’s rationale for marriage as a holy commonwealth.
Here is one source for my own Reformed, Protestant perspective on this issue.
Worldly Saints by Leland Ryken.
“The Puritan doctrine of sex was a watershed in the cultural history of the West, The Puritans devalued celibacy, glorified companionate marriage, affirmed married sex as both necessary and pure, established the ideal of wedded romantic love, and exalted the role of the wife.
This complex of ideas and values received its most eloquent and beautiful expression in Milton’s picture of the married life of Adam and Eve in his epic Paradise Lost. In portraying the perfect marriage in book four, Milton went out of his way to show that Adam and Eve enjoyed sexual union before the fall. As Adam and Eve retire to their bower for the evening, we read,
‘Straight side by side were laid, nor turned I ween
Adam from his fair spouse, nor Eve the rites
Mysterious of connubial love refused:
Whatever hypocrites austerely talk
Of purity and place and innocence,
Defaming as impure what God declares
Pure, and commands to some, leaves free to all.
Our maker bids increase, who bids abstain
But our Destroyer, foe to God and man?’
Having dissociated himself from the [Roman] Catholic tradition, Milton proceeds to give his famous apostrophe (address) to wedded love:
‘Hail wedded love, mysterious law, true source
Of human offspring, sole propriety
In paradise of all things common else.
By thee adulterous lust was driven from men
Among the bestial herds to range, by thee
Founded in reason, loyal, just and pure,
Relations dear, and all the charities
Of father, son, and brother first were known
Far be it, that I should write thee sin or blame,
Or think thee unbefitting holiest place,
Perpetual fountain of domestic sweets,
Whose bed is undefiled and chaste pronounced.’
All the usual Puritan themes are here: the Biblical basis for affirming sex (as evidenced by several key Biblical illusions in the passage), the differentiation between animal lust and human sexual love, the domestic context into which sexual fulfilment is put, and the romantic overtones of the passage. This, and not the modern stereotype, is what the Puritans really said about sex.” (Worldy Saints)
“[Husband and Wife] may joyfully give due benevolence one to the other; as two musical instruments rightly fitted do make a most pleasant and sweet harmony in a well tuned consort.” (an anonymous Puritans source, Worldly Saints, p. 44)
Richard Baxter is also a good primary source for a Protestant and Reformed companionate view of marriage and married sexuality.