Answer on bolded question: sex is gift from God. Especially it’s pleasure. In biological terms, the sensation is completely useless, apart from bringing enjoyment to married couple.
You are right, but only in part. I think, even more so, that the pleasure in sex is a means to encourage couples to have children. I would think that few people would engage in intercourse if it were not pleasurable and consequently, few people would have children.
That said, sex and its pleasure are indeed gifts from God and St. Augustine certainly agrees with us:
“Truly we must consider, that God gives us some
goods, which are to be sought for their own sake, such as wisdom, health, friendship: but others, which are necessary for the sake of somewhat, such as learning, meat, drink, sleep, marriage,
sexual intercourse.” (
e, Section 9Of the Good of Marriag)
Where our agreement breaks off is if you deny that there is anything wrong with the pleasure. There certainly is and it has a lot to due to concupiscence.
In broaded terms, it is small gift compared to salvation and faith. But the fact that something so banal provides so much joy is another testament of good Lord’s mastery and care for us. Of course, we should use this gift accordingly.
As shown above, what you say is good and I and Augustine agree with it (at least in part). But it is important to note that we don’t have full control over our pleasurable sensations. We have a craving for pleasure and as such, there is a “hole” for lust to creep into whereby a married couple can use each other merely for the satisfaction of that urge. And that is undeniably a sin.
However, can you please answer my question: do you think bishops of the Church had to develop the idea of unitive sex for several centuries? I mean, they were surrounded with layman who knew this, and preceded by Jews who did not hold such teaching?
Jewish teaching was imperfect on many levels. Christ came to perfect it. As such, just because the Jews taught such and such on a particular subject does not give it a blank check of acceptance (divorce laws and polygamy are cases in point), especially when it does not deal with the subject of God directly.
And if you are including among the laymen who knew of the unitive nature of sex those who fed their children to lions and abandoned their children to die and used potions to induce sterility, then I would say that things do not look very good. Remember that there were no modern artificial contraceptives then. The emphasis on procreation in the sexual act seems, at least logically, to be a way to counter this attitude and bring about nurturing a society where children are loved and appreciated as gifts from God.
I am constantly doing my homework too

And I am not sure how far can we go in saying Blessed Pope is Augustinian in origin. For Augustine, concupiscence is any sexual arousal, including the one done for procreation. Theology of the body would probably object such idea.
As I’ve mentioned before, St. Augustine uses the word “concupiscence” more freely than the official Church. In his use, it is not always bad. If I have St. Augustine understood correctly, the concupiscence of the evil sort is not sexual arousal itself. It is the sexual arousal is not subject to reason.
“It [sexual desire] shows, moreover, that it must be excited, not by a free choice, but by a certain seductive stimulus, and that on this very account it produces shame. This is the carnal concupiscence, which, while it is no longer accounted sin in the regenerate, yet in no case happens to nature except from sin.” (On Marriage and Concupiscence, Chapter 27).
I see nothing in here that Theology of the Body would disagree with. In fact, I know that I can find a quote from Pope John Paul II’s talks that are similar to what St. Augustine says above.