Modern debates about marriage often suffer serious flaws:
1.) The muddling of scriptural or canonical accounts of marriage with natural ones. For example, Scripture sees the relief of concupiscence as a legitimate purpose of marriage. This allows old persons, and even infertile persons to marry, so long as they are capable of copulation (see here, objection three, with the usual caveats about the supplement to the Summa). This doctrine of relief is tied to a deeper principle- sc. the fall of man. On a natural account of marriage, one has no access to the doctrine of the fall, and so it is unclear how one is supposed to deal with questions of who can get married if they can’t have children
2.) Extreme reticence to admit the reality of gender roles. Regardless of whether one follows the light of nature or the light of revelation, the traditional account of marriage cannot be defended apart from gender roles. Marriage essentially involves a shared life. If we insist that this same shared life must also be between men and women, then there must be something in this shared life proper to men as men and women as women.
This is the point on which traditional justifications of marriage are weakest.
3.) Failure to appreciate the way in which reason can solve moral problems in the face of doubt. On the traditional understanding, deliberate non reproductive sex perverts the sex drive in much the same way that joining a gang perverts the desire to belong to a group. Imagine the difficulty in persuading a gang member- especially an articulate one that has cultivated his justifications- that their desire to belong to a gang is distorting their desire to belong to a group.
4.) Reducing natural law explanations of sex either to “the person” or to the nature and structure of the sexual organs. The first is too broad and distant the from relevant factors, the second too narrow and removed from the moral good. The middle ground is the reproductive soul, or in our case, the human soul as reproductive (pay particular attention to the second paragraph in the link, whichspeaks to purposes). What food is to the individual man as an individual, sex is to the individual man as a man (or a sort of thing). Both manifest a personal desire to exist i.e. to live. discussions of procreation and “procreative and unitive acts” usually fail to show procreation is an aspect of our personal desire to exist.