6) Masturbation is a problem in
many marriages. Men
that get used to masturbating may find that easier and more gratifying that the act of intercourse. If a man get's used to a gripping motion, as oppose to vaginal sex, obviously the gripping motion can be held tighter on their person, If that's what he's used to, it might be that regular sex is not as satisfying.
As i said, there are many reasons, not all of them here.