Here's the reality: if the tactic (asking for a reduced rate) wasn't successful a reasonable percentage of the time, guys would not bother doing it.
So don't blame a guy for asking. Blame your sisters for agreeing to discounts and thus subjecting you to the requests.
Similarly, guys, don't blame ladies for using fake pics. Blame your brothers for consistently thinking with their little head and not saying thanks but no thanks. If fake pics didn't work much of the time, providers wouldn't use them.
And one can apply this logic to a whole host of other practices. Namely, if it wasn't generally acceptable or tolerated by many in the industry, it wouldn't be an issue.
I agree with awl4knot in that I suspect bartering changes the enthusiasm and quality of the session, so it isn't politeness that keeps me from bartering but my desire to get a quality session. I pick my spots with other purchases as well and ask myself will negotiating price reduce the quality of the service I receive?