I was raised Catholic and always questioned why priests and nuns could never get married. It's not normal as humans to not love someone of the opposite sex, much less to never have sex . They are against birth control, so the only reason to have sex is to have children, wtf? I guess that when a woman can no longer bear children, her and her husband should no longer be intimate, whatever!
It goes back to Paul and 1 Corinthians 7:8
"Now to the unmarried and the widows I say: It is good for them to stay unmarried, as I do."
The idea is that you are "married" to the Church and theoretically you will be more dedicated to the Church if you don't have a family to worry about. But there is more to it than that. Priests and nuns are supported by the Church, and if they were allowed to marry then the Church would be obligated to support their whole families which would cost considerably more money.