And I mean, like, also, let's consider what the other options are, anyway. So you're getting harassed. You can try...
- escalating the situation by reacting aggressively/in a confrontational manner
- ignoring it
- de-escalating by smiling, etc.
What are your other options? Escalating by bringing other people into it?
In other words, women smile because it's possibly the best option out of a pile of shit options.