You make a point, but the problem is that the only reason a lot of men will accept is "I have a boyfriend." Women will get harassed non-stop unless they use that.

"Sorry, I'm not looking for anyone right now." seems like a challenge to men. Being polite and trying to move to someone else or end the conversation doesn't work. Only the boyfriend excuse, and sometimes not even then. It's a strange paradigm that a woman can give any number of reasons and they won't stop, but saying you have somebody else does work. I don't know if it's out of fear or out of respect for another man more than a woman's choice or the respect for the sanctity of another relationship, but it would certainly be frustrating as a woman if the only way you can get out of a situation of being hit on is by bringing up another man.

I don't know that saying "sorry, I'm not interested" is necessarily an affront to the man giving attention, either by way of their looks or personality. Sure, it could be given tone or body language, but it can be done politely as well. I actually have used the "sorry, I'm not interested" to women in bars before. Of course, it's phrased a little differently, but I don't mind using "I have a girlfriend" either, because, unlike what women face, the entire world I live in isn't controlled and dominated by the opposite sex. I get to have autonomy in that situation, and they don't. Additionally, any reason I give to women will usually work, while men are persistent little fuckers more often than not.