JW's are not "In Christ" since official WT teaching is that Jesus is not the Mediator for average members. They are not born again by the Spirit of God, only the anointed are according to the WT.
if any man have not the Spirit of Christ, he is none of his - Romans 1: 9
So, your parents advice would actually prevent a person from marrying a JW .
I realize that this is not likely to go over very well.