Now Zachariah and Elizabeth were both descended from Aaron and are thus both from priestly families; they were, as the sermon said, people of some degree of importance. Mary, scripture also tells us, was kin to Elizabeth, and thus it is reasonable to believe that her social position was not too far off from that of the elder woman. Joseph perhaps could be held as having a lower status, but calling him poor is, I think, an exaggeration. A carpenter is a man with a skilled trade, not an unskilled laborer; also one must recall that he and Mary came to Bethlehem prepared to pay for lodging at an inn. One may reasonably number the newlywed carpenter outside the wealthy, but I do not think that scripture supports numbering the couple among the poor.
Nor, I think, does the unwed mother analogy truly obtain. Let us first address Mary's age. It is commonly supposed these days that Mary was barely post-pubescent, but again without scriptural warrant. This is extrapolated from medieval Ashkenazi practice and from the pattern of medieval nobility, but I do not believe this has ancient testimony, and in medieval Europe the pattern in the working and middle classes was for delayed marriage in order to accumulate the assets needed to set up a separate household. But in any case Mary is most conspicuously not a fallen woman-child abandoned by the child's father; indeed, quite the opposite happens, with Joseph taking up his fatherly duty to Mary and the child with a shove from the Spirit) even though the child is not of his seed. Thus, while we may take the prosperous interiors beloved of painters as something of a fancy, there is really no reason to take the holy family as anything other than a decently prosperous working class household, neither rich nor poor, and largely unstained by the peculiarities of its origin.
Nor do I think that there is any great contrast intended between the two mothers. The meaning is more found in the knowledge that Zachariah is of Levi (and indeed of Aaron himself), while Joseph is of Benjamin, and more precisely of Jesse's and David's line. John is therefore priestly, and Jesus kingly. Zachariah is skeptical, and Mary is not; Elizabeth's barrenness signifies, as does Mary's virginity. It is a great temptation to turn every point of scripture into some life lesson; and for an Epsicopalian these days, the lesson is often as not about social justice. But I do not see how the visitation gives such a message, and in any case, the text is plain enough that the story is about their tie: their common, strange situation in which these two unexpected and miraculous pregnancies places them together.
It is not the second great commandment which we are brought to here, but the first: to love God with all our heart and soul and mind. And in this case, our love is carried out through our acceptance of the miraculous grace by and through which we were given an incarnate savior. Mary said, "be it done to me," and Elizabeth said, "blessed is the fruit of your womb." May we also accept the advent of our savior with such joy and humility.