[happytoexplain]

The disrespect of "step it up" is derived directly from what people are being asked to step up to. The phrase by itself has no context.

I'm not following your second paragraph - it sounds like you're simply saying that the female and male version of the dress code in question have no differences in terms of respect and reasonableness in-context, but surely that's the crux of the disagreement? The point is that I find "wear a short dress and heels" to be more disrespectful than "wear a blazer/slacks/suit". I am making no comment on how those two things are comparable in other contexts, e.g. whether or not they are members of the same dress code by definition. I can't follow how that relates to respect.