The Church has always taught that sex must be unitive and procreative. I don’t think that’s the question.
What you’re asking is how the Church interpreted this truth and set it forth for the people, which in reality hasn’t changed that much. First of all, I would recommend reading Love and Responsibility which explains this very idea.
I’m not a Church History expert, but maybe I can shed SOME light.
In the first instance, the Church focused primarily on the procreative aspect, believing that one should always have sex “intending” to conceive.
(I don’t know what you mean “without the positive intention”).
The Church now, however, has a more wholistic approach to sex, both procreative and unitive. But I can tell you right now that it is A SIN to have sex with the positive intention NOT to procreate. For one would be focusing solely on the unitive and not the procreative, which is wrong.
Rather, the general understanding of today’s teaching on sex is the somewhat like the following: “I may have sex with my wife at any time (even those times which she is unlikely to conceive) beacuse I am open to life. I look foward to having a child, but if this time would not be the best time for a child for grave reasons, i must practice abstience.”
Ideally, we would have families of 5… 7… 10. But we know that isn’t possible for most people today. On the other hand, it would be a very bad thing for a couple to not have sex all the time because they’re too afraid to have a child.
Pope John Paul says something to the effect that nature has provided man with a way (menstural cycle) to have some control over how many children he has. We can use nature, in this way, to space out children, but in times where having a child would not be good at all, abstience would best be practiced.
At all times, though the intention to have a child doesn’t have to be primary, a couple should not be opposed to having a child.