This time not a Reformer, but a Catholic, albeit one beloved by the Reformers -- Augustine of Hippo, as found on BaylyBlog:
And why has Paul said: 'If he cannot control himself, let him marry?' Surely, to prevent incontinence from constraining him to adultery. If, then, he practices continence, neither let him marry nor beget children. However, if he does not control himself, let him enter into lawful wedlock, so that he may not beget children in disgrace or avoid having offspring by a more degraded form of intercourse. There are some lawfully wedded couples who resort to this last, for intercourse, even with one's lawfully wedded spouse, can take place in an unlawful and shameful manner, whenever the conception of offspring is avoided. Onan, the son of Judah, did this very thing, and the Lord slew him on that account. Therefore, the procreation of children is itself the primary, natural, legitimate purpose of marriage. Whence it follows that those who marry because of their inability to remain continent ought not to so temper their vice that they preclude the good of marriage, which is the procreation of children.
And elsewhere (cited in the comments):
It is, however, one thing for married persons to have intercourse only for the wish to beget children, which is not sinful: it is another thing for them to desire carnal pleasure in cohabitation, but with the spouse only, which involves venial sin. For although propagation of offspring is not the motive of the intercourse, there is still no attempt to prevent such propagation, either by wrong desire or evil appliance. They who resort to these, although called by the name of spouses, are really not such; they retain no vestige of true matrimony, but pretend the honourable designation as a cloak for criminal conduct. Having also proceeded so far, they are betrayed into exposing their children, which are born against their will. They hate to nourish and retain those whom they were afraid they would beget.