Disclaimer, and thoughts:
1) The views and opinions expressed in this article are not necessarily my own;
2) I do not smile at those who are sexually harassing or in other ways threateningly approaching me. HOWEVER, as I read this article I asked myself, "What do I do? In what situations am I unwillingly approached?"
A: My default is to stonewall and ignore the person ogling me. If, despite or as a reaction to this, their behavior escalates, I then attempt to leave the situation somehow. I am luckier than the author in this because some of the situations she describes are ones she can't leave - waiting for the bus, riding public transport, etc. I don't know what I would do then.
B. The exception is that if I'm on foot and someone is catcalling me from a vehicle. Then my default is to flip them off. This has backfired on me exactly once, but that backfire did result in a nasty fight between the driver of the car and my boyfriend at the time. It seems the risk of a nasty reaction is low but when it happens, boy, it's gonna be a doozy probs.
C. I've had way less of these encounters turn to threats/violence than the author has. I'm guessing she lives in a more city-like environment.
3) This issue seems very clearly related to both
a) POC and hair touching
b) Tattooed/body modded people and skin/mod touching
Holy strawman, batman. We smile because it's a non-threatening gesture meant to de-escalate conflict. We smile at bears. We smile at angry dogs. Women smile at harassers because if the smile works in drawing down the problem then they don't have to signal in other ways (making eye contact with others, backing away, withdrawn posture, etc).
Likewise, when a man sees a woman smiling at another man, he assumes the situation is under control and not his problem. I've been that guy that got between the drunk abuser and the girl that was physically trying to get away. I got no thanks. It's a hell of a line to cross before I White Knight into a situation i don't understand because a third party never de-escalates.
The fundamental problem is we're attempting to maintain a society of gender equality under social mores of gender imbalance. Men have the physical agency, women the emotional and it's bloody complicated and confuses everyone. It's not a problem that should be fixed, but it's a problem that needs to be considered because there will never be perfect communication across the gender divide for the simple reason that a lot of gender relations are based on conflict.
No dispute - men rarely get sexually harassed while out and about. But that doesn't mean men don't get harassed. I wish we could make it all go away but we can't.
So just because your boyfriend doesn't understand what you're doing, don't assume the rest of us don't.