This post continues the response to the question, what has changed regarding christians' and the Church's view of marriage and marital relations, a question raised in a comment on the post Married Saints and Continence.
Traditional View (Systematized by St. Augustine)
St. Augustine understands sexual intercourse to be so ordered to children, the "one honorable fruit" of intercourse, that even a spouse who desires sexual intercourse more than necessary for procreation, unless they do so for the sake of their spouse and their relationship with their spouse who desires such intercourse, is guilty of a venial sin, inasmuch as they are unduly attached to the pleasure of sexual intercourse or something similar, as manifested by their use of sex apart from the end for which it is given.
This continence is more meritorious; it is no sin to render the marital debt, while to demand it beyond what is necessary for begetting children is a venial sin." (St. Augustine, On the Good of Marriage, n. 6)
These goods that are necessary for the sake of something else, if someone uses them for some other purpose than that for which they were instituted, he sins, in some cases venially, in other cases mortally (ibid, n. 9).
St. Thomas basically takes the same position, though he notes that one spouse ought to have marital intercourse not only when the other spouse explicitly expresses a desire for it, but implicitly shows that he or she wants it. (In IV Sent., dist. 32, q. 1, a. 2, qa. 1) Moreover, he says that intercourse which happens to be sterile (as opposed to intercourse intentionally sterile) is not a sin, and this includes not only cases where the spouses do not know that the intercourse is sterile, but also the cases where they know it (Summa Contra Gentiles 3, 122). This may possibly show that the procreative intention for St. Thomas need only be a fundamental and habitual intention, not an actual intention in the sense of actually expecting, with at least some small probability, a child from the particular act of intercourse. (It is not certain, as it is also possible that when he describes this act as not a sin, he means that one can consent to the act [as when one's spouse desires intercourse]).
Changes definitively made through recent Church teaching
The position systematized by St. Augustine can be, with variations, roughly described as the majority view in the West until some time after St. Alphonsus Liguori. Nonetheless it was not a universal position.
In magisterial documents in the 20th century, the Catholic Church has, in an authoritative way, somewhat qualified the manner in which the principal end of the marital act has to be in the intention of the spouses. The marital act must remain intrinsically directed towards procreation, and this intrinsic order of the act to procreation must be respected by those who choose to engage in martial intercourse, but the marital act need not lead concretely lead (even in terms of probability) to the procreation of children. Having marital intercourse for the sake of the relationship between the spouses can be morally good, even if children are impossible, and neither of the spouses expects or intends to have children through that act of marital intercourse.
Each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life. (Humanae Vitae, n. 11)
If there are well-grounded reasons for spacing births, arising from the physical or psychological condition of husband or wife, or from external circumstances, the Church teaches that married people may then take advantage of the natural cycles immanent in the reproductive system and engage in marital intercourse only during those times that are infertile, thus controlling birth in a way which does not in the least offend the moral principles which We have just explained…. When the infertile period recurs, they use their married intimacy to express their mutual love and safeguard their fidelity toward one another. In doing this they certainly give proof of a true and authentic love. (Ibid., n. 16)
Common Change of Viewpoint
The common view, in contrast to the position of the Church, is now that sexual or marital intercourse, or at least individual acts, need not be ordered in any significant way to procreation. This is the complete opposite of the traditional position systematized by St. Augustine. This view has a number of consequences. One of these is that it leads quite naturally to the abandonment of any general moral objection to homosexual unions, even if it still allows for various sound political and religious reasons to not recognize them as marriages.