There is a common saying that those who leave do so because of sin. That they either have committed a sin and want to justify it, or wish to commit a sin and leave so as to avoid facing the consequences of their evil desires.
Research shows this is untrue. Whenever a poll is taken of those questioning their religion, who are on the way out or have already left, wishing to engage in different behavior is rarely a motivation. They are often people who were very devout, but who couldn’t reconcile the contradictions they saw. Even when their behavior changes it is usually after they have lost confidence in their religion to some degree.
The most major problem with the idea that people leave because of sin is that it is an idea not supported by the Christian message: which says that we are all sinners, and that religion was created for those who are sinful, not those who are righteous. The New Testament rationale for forming a church is to welcome and treat sinners, and a faith that fails to do this would be failings its primary mission.
Obviously some people go through some very un-Mormon behavior, but some types of un-Christian behavior are highly tolerated, but other types are greatly frowned upon. People react to the system they were in, not always in constructive ways. Perhaps they seek for dubious comforts, but the reaction from their fellow co-religionists is usually to exercise less mercy and forgiveness (from a people who claim to believe in a God who can forgive all things).
Ultimately, whatever course the passage out of religion takes, it is convenient for some leaders and followers to dismiss concerns and questions, and sideline the real issues, because of a persons imperfections (in their eyes), to consider their points invalid because the person raising them is imperfect.