JOEL - When a widow woman who already has a temple marriage gets married a second time she is married "for time only" to the second honorable man and therefore is not sealed to that second husband for eternity. Any children she has with the second husband would be sealed to her and her first husband. Her second husband would still be sealed to his first wife and children(if he had them). However, if the woman has the sealing to her first husband canceled, she may then be sealed to the second husband for time and eternity and any future children they have will be sealed to them.
Actually, in terms of temple work for the dead a deceased woman can be sealed for eternity to any and all deceased men that she was married to before she died, even if she was sealed to one of them while alive.
Why is it that a living man can be sealed to more than one woman, but not a woman to more than one man? As I have mentioned elsewhere in a question about polygamy, the number of righteous women probably outnumbers the number of righteous men. And at the present time there are about 700,000 more women in the church than men. So chances are there will be quite a few more women eligible for the Celestial kingdom than men. Perhaps this is God's way of evening out the odds to make sure everyone who is worthy will be sealed to someone in the hereafter. But this is just my opinion. Only God knows the real reason why He set it up this way. I have faith that He is not unfair and that He will sort things out in the end for the benefit and eternal happiness of everyone.