when people argue against a transfeminine reading of ranma, they often argue against the idea that ranma currently during the story sees himself as a girl, and thats just not how anyone who reads ranma as transfem thinks of that reading. ranma doesnt actively think of himself as a girl, but he is also someone whos parents decided when he was a child that he will be a pillar of masculinity or kill himself, and while he may not have known of about the suicide part, he was raised by a misogynist and being constantly on the move left him pretty isolated. Ranma is also someone not very aware of and, even when he is, not very honest about his emotions. you see it in how he struggles and refuses to express his feelings for Akane, or in how he has to filter his friendship with Ryoga through the language of rivalry.
And despite all of that, he just so obviously has fun when hes a girl, in ways he doesnt as a guy. Hes no less comfortable fighting as a girl than as a guy, but he doesnt let himself be as vulenrable with Akane as a guy, and he only has like three outfits as a guy but past the first time Nabiki and Kasumi dress him up as a girl, he doesnt hesitate to dress up as a girl on his own. Even when he is trying to cure his curse, he still has fun as a girl. When he and Shampoo did the Kew-Pid sisters bit to get the water proof soap from Ryoga, Ranma had way more fun doing that than Shampoo.
Which brings me to the second point often brought up against Ranma being transfem. He does try to cure his curse, but id argue trying to rid yourself of femininity that you obviously enjoy so that you can better perform the role of masculinity expected of you is a fairly common transfeminine experience.
and thats not even getting into how the various gengered ways much of the cast (mis)treats ranma is, intentionally or not, just how people treat trans women. id agrue its most blatant with Happosai. Happosai refuses to take Akane as a student because he will only take male students. His previous students, Genma and Soun, tried to bury him alive after years of being treated as his punching bags and lackeys and being forced to clean up his messes. Many of those messes are Happosai constantly harassing and assaulting women in various ways, which now includes all of the Tendo girls. Ranma gets abused both ways by Happosai.
Even if Ranma isnt textually a trans woman, he, whether intentionally or not, represents one.