I had a similar discussion with my Elders Quorum president and I was told that men can be sealed to only one woman. If he becomes a widower, he can be remarried but only for time, not eternity. Likewise for a woman who loses her husband- she can remarry but if she was sealed to her previous husband, her marriage would be for time only. I too have heard the "urban legend" that polygamy is practiced in heaven but I have yet to find any doctrine or scripture to support that. I have read and heard many comments attacking the LDS faith; that's the problem with ignorance- it propagates much quicker than truth in today's world.