Holy fuck imagine thinking you're not allowed to even communicate with the other sex while in a relationship. Like what the fuck you can't make small talk and chat with the opposite sex just to make life easier and better?
What strange creatures you are.
I said that going beyond what was necessary in day to day life with the opposite sex when you're in a relationship is pointless. You're misreading and putting ideas there that haven't been expressed.
The only thing that active friendships with the opposite sex leads (while in a relationship) are feelings of not being as interesting, valued, or loved. It will also almost inevitably lead to some form of cheating, not because your partner is disloyal but the other person involved is a snake.
Think about this: you're in a relationship with a girl, and you ditch plans with her to hang out with your female friend. How do you think that makes your girlfriend feel? No amount of "just friends" rhetoric is going to change the fact that this type of behavior makes your girlfriend feel like she's not as valuable to you as your "friend"
Similar feelings happen when a guy "friend" gangs out with your girlfriend. When the opposite sex shows interest in one another, the majority of the time it's because they're either sexually attracted to then, or at least think that they're pretty/handsome, which will eventually develop into a desire to see them naked.
If a girl is all about hanging out with a guy, chances are she wants to fuck him.
If a guy is always trying to hang out with a girl, chances are he's trying to fuck her.
Especially if they insist on hanging out alone, without you/others involved, it's most likely because one of them is trying to find an opening where a normally loyal person is vulnerable and take advantage of them.
Hanging out with the opposite sex, unless your S/O is involved all the time, creates uncesssry drama that can't be avoided by any amount of rhetoric or feelings.