"Thats why doors are left open, even with college teachers talking to students in their offices."
It's a really good idea. Billy Graham, in his wisdom, never allowed himself to be alone in a room with a woman who was not his wife. It was a rule he followed very carefully.
No accusation of untoward contact was ever made against this great evangelist. He was a wise man.
i have always taught my kids this principal, but have not limited it to members of the opposite sex. when they have friends over, regardless of who they are, the door has to remain ajar, and i frequently walk in unannouced. i constantly impress on my 19 yr old the need to never be completely alone with any of his female friends, especially the girls who are younger than he is. i teach it as "avoiding even the appearance of evil" in that no one can ever accuse him of inappropriate behavior towards a young lady.
as far as the open doors with friends, when i was in jr high i used to spend the night with a friend who molested me at night (behind closed doors) while her mom was in the next room. i never want my kids to go through that!