Suppose there is a group of girls at a high school is approached by a single guy that doesn't know them. What is the odds of being welcomed into the conversation without any questioning looks? At least at the high school I went to, it'd pretty low odds. As a consequence, guys learned not to do that.
That stuff carries forward, even if the situation has changed somewhat. You have a population of men trying to be polite. They're not going to approach groups that have given them negative feedback before, because they don't want to be a jerk.
