Interesting thread. I'm not the sort of person who normally spends much time thinking about public feeding, in simple terms it's none of concern and none of my business, but I'm also unsure how the social etiquette angle is supposed to work. Not in a theoretical support/criticism sense but in a practical everyday guideline sense.
Based on what I've read you're not supposed to look in the direction of someone nursing in public or to make it obvious that I'm looking away or show virtually any decipherable reaction at all. To be perfectly honest that's just not how my brain works. I happen to believe that most men are fundamentally simple creatures. It takes us a moment to figure out what we're seeing and we often wear our reactions on our sleeve. Our brains are instinctively drawn to certain sounds, movements, and anatomical shapes, whether we want them to be or not, and it takes a few seconds to reach the point where we can decipher and react (or pretend to not react) to what we've inadvertently witnessed.
Here in Texas we're probably a generation or two away from simply not noticing and in the meantime the transitional period is genuinely confusing to me. At my office the unisex medical station was suddenly turned into a full time women-only nursing station and all the men in the office were summarily excluded. I don't want to prevent women from having a place to nurse but how does preventing men from accessing first aid items and basic medicine represent a reasonable compromise? The next solution offered was for a man to ask a woman to retrieve medicine and first aid items on his behalf. Really? I'd like to be supportive but that kind of reverse sexism nonsense really chaps my hide and probably hurts the cause as well.
There are literally hundreds of articles on how women should anticipate and respond to various aspects of public feeding, but the articles I've seen about how unrelated/unknown men are supposed to react to public feeding are few and far between. Some of the suggestions are too vague to be helpful while others range from the patently absurd to the borderline impossible. Eventually society will come to grips with what constitutes an acceptable reaction but until then it feels like a bit of a minefield for the well meaning but emotionally simplistic among us.