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I truly believe that if a man cheats he was always inclined to do so. If you think your girlfriend's resistance to being exposed to anything critical of Mormonism sounds cult-like, you are right.
She is considered "an old maid" by Mormon standards, so she may be willing to marry you--hoping you will convert someday --but she will constantly be reminded that your marriage is inferior to the "Eternal Families" of sealed Mormons, and she will fear dying and never seeing her loved ones again.
I have no idea if he stayed.
Make sure she knows the plan, so you both can dress appropriately.