If they already belonged to other men, and were other men's wives, it is still problematic.
Not at all if one understands the principle of revelation.
You use the term "normative," but the sociological institution that created marriage "norms" as observed in 19th century society was actually the Christian Church, and not any secular institution. As others have noted, there would be nothing "normative" about marrying other men's wives, or marrying multiple wives, or marrying women with intent to be celibate in this life but procreative and sexual in the next.
I use "normative" only as a term designating cohabitation and sexual relations. The LDS sealing is for time and all eternity, and nothing in the sealing necessarily involves time at all, given the existence of eternal relationships.
Further, there is nothing "normative" about the concept of "spiritual marriage," as the term is really an oxymoron in any sense of what would be considered normative.
I didn't say it was normative nor that it should so be considered. Precisely my point. Many of these wives were understood to be sealed to Joseph as a means, if they endured to the end themselves, to exaltation they otherwise might not have been able to avail themselves of in this life. With the LDS Church one of the fastest growing religions in the world, such concerns have now been obviated, but at the time, the concerns were apparently quite different.
The concept of marriage as handed down in both Old and New Testaments, and well known in the 19th century, involved a man and a woman cleaving to each other to become "one flesh." If it wasn't consummated, it wasn't a marriage. Period. There is no concept of "spiritual marriage," any more than there is a concept of "sexless intercourse."
Now, you need to go to the Old Testament and see the conditions and rules, under the Law of Moses, for plural marriage, as well as Abraham's, Issac's, Jacob's, and Moses' plural marriages. Next, you don't seem to understand that the restored Gospel of Jesus Christ, as we understand it, is not a historical derivation of Old Testament and New Testament theology but a divinely revealed restoration of both the correct understanding of thsoe concepts but of lost knowledge and new knowledge as yet unrevealed in any dispensation.
Both the Old Testament and New Testament are, in any case, heavily redacted and fragmentary records, so the fact that you do not see such and such in them means less than you may think.
Have you read the Gospel of Philip, a particularly interesting and early Christian document?