The real reason we can't go in bedrooms of the opposite sex or use the bathroom is cause it would decrease our chances of getting married. Think about it. If a guy walks into your bedroom and sees how messy or smelly it is, he might think twice. Girls, has your opinion of a guy change when you see his bathroom?
As far as curfew goes, you always want what you can't have. So if you can't spend the night with someone, you are more inclined to marry them.