100 to 11 lol.
Says it all really. Even Oda cant decide though as he had her say shes Kaidos son IIRC to Green Bull too. Kaido called her his son too at least once I believe. Its such a fucking mess.
I just think she identifies as Oden regardless of his gender and thats her gender too. if Oden was a living, talking sword, she would identify as him regardless. If he was a frog, a mink, a ship, a tree or whatever, still Oden.
The key part is that she idolised Oden, he was her beacon of hope, the ultimate rebel, the vagabond who rose up against sheer tryanny and tyrants and dared to risk his life for others. She idolised that as a literal prisoner for her entire life. Its only natural she would want to be just like him and idolise him, much like how certain characters inspire others in OP. Roger inspiring many others especially for example. Or just look at Green Bull and Akainu funnily enough too.
Since Ushimaru had a 2 sword style IIRC and so did Oden, its a HUGE missed opportunity that Oda never had try to replicate their fighting styles and be like the men who saved her (Ushimaru and his fellow daimyo or w/e) and the man who truly inspired her (Oden obviously).
Yamato presents very femininely too although I don't want to get into arguments about gender, sex and what you identify as either but speaking of typical biological terms, she is female and presents herself as that. She did bathe with the men though but ive seen very tomboyish girls/women do that too, girls who just want to be one of the boys but dont necessarily identify as male, just tomboys ofc. They still have strongly feminine qualities but they are more masculine in general.
Hell two of my closest female friends, one is a champion weightlifter/body builder genuinely, shes absolutely built like a brick or whatever, a true Amazonian and shes extremely feminine and beautiful. Think Big Mom when she was younger but much more muscular. Very tall like her too. She was very sweet, genuine, kind, funny etc, a true tomboy but still elegant, feminine and such, she had an amazing balance of the two, it was mesmerising how she did it. (Actually to think of it, she reminds me a LOT of Robin funnily enough.)
My other female friend was female at birth but realised she was trans and went all the way with it. I wasnt in contact with her when she realised as life seperated us due to her going to university in another city and being far too busy and a bunch of extreme shit happened to me so I went into severe depression and such BUT she or well he I mean seems a lot happier now as far as I know, he as (formerly she ofc) prefers to be called now, has gone one to do modelling, be involved in a hollywood film with production and such. So not bad at all lol.
My point is anyway is if Yamato was meant to be classed as male, Oda would make it more obvious like he did with Kiku saying she is a woman at heart. Yamato just seems like an extroverted as hell tomboy like some female friends Ive had including the one I mentioned, the weight lifting one and I don't feel she is trans or thus male either.
If I get buried or flak for this so be it, but I've struggled with gender/identity issues myself and believed myself trans for a while too, so I get it too.
Celestial D Dragons post above is the best, most accurate/nuanced/summarised take I've seen on it anyway, could not have said it better myself.