Aside from the Halachick issues, Issurim (if you hold that way), the entire concept is impossible.
Single Guys and Single Girls CAN NOT (not practical) be friends! All it leads to is akwardness and inner feelings being hurt.
A guy who says “my best friend is a girl”, is basically saying “I like this girl so much, but she doesn’t like me back, so we are best friends, so I can always have hope of marrying her”.
The guy who is “friends” with a girl, will ALWAYS hold out hope that they will get engaged/married to each other.
And yes the same/above applies to the girl also.