Here's an idea that I don't know if it goes anywhere. Someone with more imagination might be able to work with it though.
There is a fanon idea that Ranma specifically, or maybe just anyone in Anything Goes (Musabetsu Kakuto, whatever) must accept every challenge that comes his way. Let's suppose that's actually true instead of fanon, and then let's also suppose that Ranma or whoever can't hold back in any fight. Fights don't have to be, like, instant speedblitzes, but they can't sandbag either.
Thought. When Ranma (female) spars with Akane initially he dodges all of her attacks and never touches her, except with one finger on the back of her head after Akane punches a small hole in one of the dojo walls. In this, instead of wasting time dodging all of her attacks, he steps in on the first one and slugs her in the stomach. Hard, but only enough to wind her and maybe bring her to her knees -- hard enough to knock her out would be pointless and not hard enough to wind her probably doesn't stop the fight, so both those are essentially a waste of time.
In the ensuing conversation once Akane sees no point in continuing, as Ranma is clearly better than her, after Akane says something like "I'm just glad you're a girl. I couldn't stand losing to a boy", Ranma responds by saying something like "I don't want to be a girl".
This is of course true, and causes Akane to pause. Ranma elaborates by adding, "I was raised as a boy" (true), "I think of myself as a boy" (also true), and "I hate being a girl" (semi-debateable). None of that actually says outright that he IS a boy. So Akane gets the impression that Ranma is a very masculine girl (in mindset). The conversation continues some more, and the incident in the bath is short-circuited -- it doesn't happen, because Kasumi doesn't more or less shove Ranma (who K believes is female) into bathing with her sister. Maybe Akane never even starts that bath, doubly short-circuiting it.
This removes only one aspect of the bad impression Akane gets of Ranma and nothing else. How could it proceed?
EDIT Jeebus, I just realized how more or less dead this thread is. Shame too; so many memories ...