Nero? Caligula? Pol Pot? Stalin? Mao?
These are poor examples. All of these men's were considered evil by everyone except the people they brainwashed. The general flow of the argument is also something of a stretch. There is undoubtedly a moral absolute, but on top of this moral absolute are built a set of cultural expectations. The Mormon apologists seem to be arguing that polygamy and marriageable ages fall outside the realm of morality, and may rather be classified as cultural conditioning. In this case, presentism would apply only to that which is cultural, not to that which is moral.
Alternatively, one could argue that different generations are given different amounts of "light," and that we can judge someone only by the light they have been given. So things like slaveholding, marrying young girls, and polygamy become decidedly less punishable when they occur in a time or place where such things are "acceptable." This argument fails where God seems to ask his people to revert to a lesser-light mode, as in the case of Joseph's polygamy. If monogamy is superior to polygamy (as indeed the principles of love and fairness seem to demand), then why is God having Joseph-- who lived in a greater-light society-- revert back to a lesser-light practice? Moreover, in a society where marrying young girls was acceptable but not universal, why would God have Joseph pursue the lesser-light option rather than the greater-light option? Joseph does not seem to have been under any societal pressure to marry these girls; the only pressure came from God and angels with flaming swords.
So the second argument, it seems, faces insurmountable obstacles. Mormons then must pursue the first: that is, that polygamy and marrying young girls are actually morally neutral, and only our cultural conditioning leads us to believe otherwise. But this still leaves us in some doubt as to why God would command it so vehemently that Emma is threatened with destruction and Joseph Smith with death, and girls' entire families are promised exaltation if they agree.
Perhaps an option that doesn't rely on presentism would make better sense of the data. For example, maybe polygamy is actually a morally superior state. In this case society is simply wrong, and the LDS church's current commandment to live monogamously is simply a temporary accommodation with the culture. Or, on the other hand, maybe polygamy was a terrible mistake from the beginning, and the Manifesto was God's way of course-correcting. Both of these options have a more honest ring to them than the presentist defenses offered above; they at least take into account the vigor with which polygamy was pursued--
as doctrine, not as mere cultural practice.
It would seem to me that the principles of love and equality make polygamy, if not the worst of sins, at least morally less desirable than monogamy. While Brigham Young's "sister-wives" seem to have gotten along reasonably well, the success-stories are counter-balanced by tales of misery or-- in the case of more stoic women-- at least hints of discontentment. Polygamy seems to me degrading to the female partners. I cannot see how it could be either morally neutral or-- heaven forbid-- morally superior to monogamy. I see only one coherent way out of this for the reasonable Latter-day Saint, and that is that Joseph Smith is a man and the church a human institution, and that both therefore are prone to make mistakes, but this needn't make them any less true.
As for the question of what was acceptable in the United States in the 1820's and 30's, consider the following references. They seem to place the legal marriageable age at 12, but the actual age most girls got married at 16-20 (see the third reference).
http://books.google.com/books?vid=OCLC1 ... 0&as_brr=1
http://books.google.com/books?vid=OCLC5 ... 0&as_brr=1
http://books.google.com/books?vid=OCLC0 ... 0&as_brr=1
http://books.google.com/books?vid=OCLC2 ... 0&as_brr=1
-CK