In the book of Mormon, Alma The Younger goes forth preaching to a very wicked city of Nephites telling them that thier brothers, the Lamanites who hate them and seek to destroy them, have the mercy of God upon them, because they were taught wrong by thier fathers. That the Nephites who were brought up in the proper traditions of thier fathers who knew better will surely be destroyed because of thier wickedness.
This goes for any other group of people. God has mercy on those who ignorantly sin, and are raised in the unrighteous traditions of thier fathers. Those who were not taught the proper ways of the Lord. The sins of those are placed on the heads of the fathers and forefathers who knew better and did nothing.
LDS Memebers believe we will be judged more harshly than anyone else on the earth, because we believe have a fulness of the udnerstanding of the gospel. and with that knowledge comes great blessings, but even greater judgements on our sins, because we know better.
Those who know better will be judged according to thier knowledge. It is harder for a person who truely knew better to repent of something than someone who just found out what they have been doing was a sin.
So really, "Heaven" to LDS members is not an exclusive club, we believe that only those who partake in the blessings of the truth will be exalted in the Kingdom of God, but those who do not know the truth, will not be cast off but have a chance to learn the truth in the hereafter and still live with our Heavenly Father. Hence why we do proxy baptisms and work for the dead in Temples, so that all those who have gone on before us ignorantly have those blessings in the hereafter before the great and terrible judgement.