Quite rightly, opinions vary about whether or not men and women can truly be just friends [1] as everyone has encountered different situations from which to draw their point of view. And things only get more complicated when you’re in a relationship. 
I’ve always figured that once you’re in a serious relationship, friendships of the opposite sex would naturally fade away. Once you have a special someone it’s hard enough to keep your friends of the same sex, let alone members of the opposite. Maybe it's just a matter of the difference between a casual acquaintance and a true friend. But if so, where do you draw the line? Do you think having any friends of the opposite sex when you’re in a serious relationship is unusual? Or do you think it’s completely normal and reasonable?
Source [2]