God designed sex both for pleasure and reproduction.
I think that we need to differentiate between these two things. Yes it is pleasurable but did God design it for pleasure? It would be a strange God indeed that issued the command to go forth and multiply and who works his covenant through families to make sex painful. That it is pleasurable does not mean it is to used for the pleasure it gives. That is hedonism.
Are you saying that people who cannot have children should not get married or have sex in marriage?
The primary reason for marriage is not sex but companionship. Indeed it is the general rule that all should get married. The man will have natural sexual desires and so they must be taken care of soberly as per 1 Cor 7.
The logic of your argument - as I see it - would mean that it is wrong to enjoy having sex.
Not at all. I am saying that whilst husbands and wives ought enjoy having sex they ought enjoy it in line with which God designed it. As St. Augustine notes, the bed is defiled "not only by fornication and adultery, which are damnable disgraces, but also by any of those excesses of cohabitation such as do not arise from any prevailing desire of children, but from an overbearing lust of pleasure".
As noted above, We can be controlled by our fleshly desires and surely that would be wrong within marriage for as 1 Thessalonians 4:4 states "every one of you should know how to possess his vessel in sanctification and honour". On this verse John Gill comments:
Now for a man to possess his vessel in this sense, is to enjoy his wife, and to use that power he has over her in a becoming manner; and which is here directed to "in sanctification and honour"; that is, in a chaste and honourable way; for marriage is honourable when the bed is kept undefiled; and which may be defiled, not only by taking another into it, and which is not possessing the wife in sanctification and honour, it is the reverse, for it is a breaking through the rules of chastity and honour; but it may even be defiled with a man's own wife, by using her in an unnatural way, or by any unlawful copulation with her; for so to do is to use her in an unholy, unchaste, wicked, and dishonourable manner; whereas possessing of her according to the order and course of nature, is by the Jews, in agreement with the apostle, called, מקדש עצמו, "a man's sanctifying himself", and is chaste, and honourable. And it may be observed, that the Jews use the same phrase concerning conjugal embraces as the apostle does here. One of their canons runs thus: "though a man's wife is free for him at all times, it is fit and proper for a disciple of a wise man to use himself בקדושה, "in", or "to sanctification".
John Calvin
Moreover, when spouses are made aware that their union is blessed by the Lord, they are thereby reminded that they must not give way to intemperate and unrestrained indulgence. For though honourable wedlock veils the turpitude of incontinence, it does not follow that it ought forthwith to become a stimulus to it. Wherefore, let spouses consider that all things are not lawful for them. Let there be sobriety in the behaviour of the husband toward the wife, and of the wife in her turn toward the husband; each so acting as not to do any thing unbecoming the dignity and temperance of married life. Marriage contracted in the Lord ought to exhibit measure and modesty - not run to the extreme of wantonness. This excess Ambrose censured gravely, but not undeservedly, when he described the man who shows no modesty or comeliness in conjugal intercourse, as committing adultery with his wife. Lastly let us consider who the Lawgiver is that thus condemns fornication: even He who, as he is entitled to possess us entirely, requires integrity of body, soul, and spirit. Therefore, while he forbids fornication, he at the same time forbids us to lay snares for our neighbour’s chastity by lascivious attire, obscene gestures, and impure conversation. There was reason in the remark made by Archelaus to a youth clothed effeminately and over-luxuriously, that it mattered not in what part his wantonness appeared. We must have respect to God, who abhors all contaminations whatever be the part of soul or body in which it appears. And that there may be no doubt about it, let us remember, that what the Lord here commends is chastity. If he requires chastity, he condemns every thing which is opposed to it. (Institutes 2.8.44)