Some discussion of the primary end of marriage here. Compare with the United States Catechism for Adults.

From an article by Robert T. Miller criticizing Msgr. Alberto Bonandi:

More specifically, Bonandi argues that there are two ends of marriage—the unitive and the procreative—and if the Church permits the divorced and remarried to pursue one of these ends (the unitive), "consistency would require" that the Church also permit such persons to pursue the other (the procreative), and so to have sexual intercourse. But these "ends" are not the ends of "marriage," no matter what theologians or popes may have carelessly said; they are the ends, rather, of marriage acts, that is, of sexual acts. In contemporary Catholic moral doctrine, a sexual act is licit if, among other things, it is appropriately ordered to these two ends; that is, it is a necessary (but not sufficient) condition of the moral liceity of a sexual act that it be of a kind that is (a) fit to produce a certain kind of emotional intimacy between the spouses, and (b) fit to be procreative. Thus sexual acts incompatible with either the unitive end (some people mention in vitro fertilization) or the procreative end (e.g., masturbation) are morally wrong. The reason, incidentally, that adulterous sexual acts are wrong is that, while they may be ordered to the unitive end, they are incompatible with the procreative end because an adulterous relationship is not a reasonable one in which to rear the children that the act may produce.

Bonandi takes these ends, which govern the moral quality of sexual acts, and converts them into norms governing human relationships generally: For Bonandi, if a relationship is fostering the unitive end, then that relationship may (perhaps ought to) foster the procreative end as well. The implications of this are rather shocking. For if, as Bonandi says, a man and a woman sharing a life together and rearing children are pursuing the unitive end and so may pursue the procreative end as well, then a widower who invites his own mother into his home to assist in the rearing of his children will be pursuing the unitive end with her and so may have sexual intercourse with her as well, which is worse than absurd. In truth, we pursue the unitive end with different people in different ways all the time. This does not license us to pursue the procreative end as well.