That's why he turns into a she. Ranma and his dad are martial artists. They have been on a training journey since Ranma was an infant. Thanks to a character flaw in Genma, the dad, they go to a park in China, not realizing that it is the land of 100 cursed pools. Anyone falling in one takes on the form of the last thing that drowned in it. Ranma kicks Pop into the pool of the drowned giant panda. Astounded that a giant panda not only comes out of the pool, he's more surprised that it has his dad's martial arts style. In that confusion, Genma kicks Ranma into the pool of the drowned girl. He surfaces as a redhead and the first thing he realized, he checks out his boobs.
Once cursed, whenever one gets cold water either splashed on them or immersed in cold water, like rain or a pool, they take on the image of their cursed form. Ranma becomes the stacked redhead, Genma becomes the panda. Hot water, OTOH, will transform them back to their original form. There are a few other supporting characters that also carry the curse from other looks, but that comes up later.
Google the wiki on it and you'll see a strange romantic comedy evolve. I got interested in it because a friend does house clean outs and got season two box set. I picked it up for $5 because my grandkids are anime fans. I laughed my ass off after reading the wiki on the show and watching it. I've been buying a season a month. Need season seven, the OVAs, the movies, and the manga. So much better that the hentai videos I have. Now that's some weird shit there.