Realizing that there may be husbands or wives left behind that did not accept Christ before the Rapture, does that mean if one spouse accepts Christ and is raptured and the mate accepts Him in the Trib, that they will not be together in the Millennium because the mate cannot go to the New Jerusalem? If so, there are surely going to be big regrets in the Trib when they realize they missed it.
According to my understanding of Rev. 21:27, your conclusion is correct.
Nothing impure will ever enter it (the New Jerusalem) nor will anyone who does what is shameful or deceitful, but only those whose names are written in the Lamb’s book of life.
The spouse who goes in the rapture will receive a resurrection body and dwell in the New Jerusalem. The one who waits until after the rapture to accept the Lord and then survives the Great Tribulation will enter the Millennium in his or her natural state and will not be allowed to enter the New Jerusalem, since nothing impure can ever enter it.
Restrictions imposed upon a spouse who becomes a tribulation martyr and is resurrected at the 2nd Coming are less clear, although Rev. 21:27 also tells us only those who’s names are written in the Lamb’s book of life can enter the New Jerusalem. Many scholars believe only the names of those in the Church are written in that book. If so, even resurrected believers who are not in the Church would be barred from entering.
However, I haven’t found any indication in the Bible that Church age believers would be prevented from visiting Earth. Many of us have friends and family who will become Tribulation believers. I think our disappearance will most likely be the event that causes them to take that final step, and whether they survive or are martyred we will have the opportunity to see them again.
We should remember that our relationships will change after the rapture. The husband and wife relationship as we know it will be no more, but will be replaced by the kind of relationship the angels enjoy (Matt. 22:30). This will be true whether both husband and wife are raptured or whether one or the other is left behind.