Propriety matters, even if it's an old fashioned Victorian age custom.
When your marriage is doing fine, both of you are happy and content, then being around "friends" of the opposite sex is not harmful.
However, anyone who's been married knows there are valleys too, and at those times when things don't go well we tend to romanticize or even worse sexualize friendships into something they are not. Soon, pangs of lust pop up, or genuine feelings, and the partner begins to replace you with him.
I used to counsel, and I never found a case of infidelity where it wasn't innocent to begin with.
Men and women can be friends, absolutely, but if you're friends with someone long enough one of you will want to hump the other one. And, in most cases, the humping occurs eventually if given the nurishment that it needs.