This can't be happening all that often; wives typically outlive their husbands. It really only makes sense for a man to be sealed for eternity to more than one deceased wife if there is not going to be an exactly equal number of both genders who prove worthy of the blessings of eternal life and exaltation, the foundation of which is temple marriage for time and all eternity. It is more a common supposition than an actual doctrine that there will be more women who prove worthy of these eternal blessings than men who do so. A simple look at the world we live in, to see who is sweet and faithful, compared to who is crude and evil, with the implications this has for eternity, unfortunately seems more often to reflect favorably on the daughters of God than upon His sons. I only offer this for what it is worth; it accepts that God (as evidenced in the Bible, notably that from Jacob and his four wives were created the twelve tribes of Israel) has provided that at times plural marriage has a proper purpose on a limited basis, and this may be one of them. The real point is that no one, male or female, who proves worthy of the blessings of eternal marriage and family in the celestial kingdom of God will be left out, for any reason.
To clarify another point which has arisen within the answers to your question, there is indeed one circumstance where a deceased woman may be sealed to more than one man. It applies only if she had more than one husband, but she and they died without anyone knowing her preference, as for example if they lived when the temple ordinances were not on the earth, or they did not yet know about these things, or even if they were members of the Church, but none were sealed together and we still don't know the correct combination to seal. Since no one now knows what her preference would be, the respective families may have her sealings performed relative to each husband, or the Church itself may through its extraction (now called indexing) work, but then the choice will be hers as to which one of these sealings will be binding in eternity. So she really isn't forever sealed to all, but her option for her own best choice is thus covered. This just makes sense, so this policy was in fact adopted. If a deceased wife had more than one husband, but was sealed to one of them during her lifetime, it is considered already decided, assuming she is satisfied.