In the cycle of my reading list, I’ve finally come back around to The Feynman Lectures on Physics and have been working through Volume II, Mainly Electromagnetism and Matter. An off-topic note for the wide margins of the page came to mind while reading the following paragraph (emphasis added):

We have gotten the following interesting result: If we go high enough in frequency, the electric field at the center of our condenser will be one way and the electric field near the edge will point in the opposite direction. … At the edge of the plates, the electric field will have a rather high magnitude opposite the direction we would expect. That is the terrible thing that can happen to a capacitor at high frequencies. If we go to very high frequencies, the direction of the electric field oscillates back and forth many times as we go out from the center of the capacitor. Also there are the magnetic fields associated with these electric fields. It is not surprising that our capacitor doesn’t look like the ideal capacitance for high frequencies. We may even start to wonder whether it looks more like a capacitor or an inductance. We should emphasize that there are even more complicated effects that we have neglected which happen at the edges of the capacitor. For instance, there will be a radiation of waves out past the edges, so the fields are even more complicated than the ones we have computed, but we will not worry about those effects now.

Repeatedly throughout the lectures, Feynman notes where the equations and principles on which he’s expounding represent ideal, not necessarily real or even possible, circumstances. We codify those ideals because, for a range of practical applications, they are “close enough” and because the basic rules are preliminary requirements for understanding subsequent adjustments and variations. There’s something similar in Platonic and Augustinian philosophy, wherein an ideal of everything exists in another dimension (in God), and we take advantage of such abstractions in order to understand how the world works and to establish baselines from which to progress.
My marginal note had to do with a letter by Vivian Olsiewski Healey in the latest issue of First Things (not online):

Bodily Union is very important, but it is arbitrary to assume that it is more important than any other element of marriage. Should a paraplegic who cannot perform sexual intercourse be denied the right to marry? People with such disabilities are married within the Catholic Church, which shows that bodily union cannot be isolated from the other aspects of interpersonal union.
Once we accept that many couples married in the Catholic Church do not live up to the ideal of complete sacred unions of bodies, minds, and spirits, we will see that the sanctity of marriage is not bassed solely on physical union.

One often hears a sort of negative variation of this argument, pointing to deficiencies in individual marriages or deviations from the ideal as evidence that the institution ought to be redefined to exclude the requirement of opposite sex, because it’s more about the spiritual connection than the physical. To fulfill its social function, though, marriage must maintain a plain, ideal definition to which individuals make their adjustments, and erasing the bodily union of a man and a woman in the person of their child changes the institution’s intrinsic nature.
Built according to plan, a strong marital culture will have positive effects at its edges and beyond. The existence of those effects, like the imperfections in its core, do not justify refashioning according to different ideals.