The two biggest reasons imho are
1) Wanting to find someone to share eternity with.
2) Protect the kids from false teachings and influences of the unbeliever.
Valid reasons imho.
Just one more reason the LDS church remains a major antagonist in the war against families.
No, they merely promote the ideal family.
Ironic considering how the church tries to sell itself as family friendly. Nothing is more important to a TBM than having an eternal family. Any relationship that threatens that needs to be cut off.
The Church actually does a pretty good job of allowing people to come to the same conclusions that match the teachings of Jesus such as:
34 Think not that I am come to send peace on earth: I came not to send peace, but a sword.
35 For I am come to set a man at variance against his father, and the daughter against her mother, and the daughter in law against her mother in law.
36
And a man’s foes shall be they of his own household.37 He that loveth father or mother more than me is not worthy of me: and he that loveth son or daughter more than me is not worthy of me.
38 And he that taketh not his cross, and followeth after me, is not worthy of me.
Matthew 10
Those who do not come to the same conclusions usually fall away.