The way I have seen it put before is if within the friendship of opposite sexes, either party discusses something they wouldn't want their SO to know about, then the friendship is inappropriate.
Saffie me 46 H 46 M in 1986 D20,D18,S16,D13 H's A 01/05 to 07/06 H recommitted to M 07/06 renewed vows 09/06 Going from strength to strength
