I was speaking to a Japanese friend of mine, and this is the info I got
on the topic of a guy taking a girl's family name.

"Well, if the guy isn't the first born and the family to which he's 
marrying into had no sons, then they'll 'adopt' a man and then have him 
marry the daughter. It was that way for my father."

Me: Well, someone said that Ranma may take Akane's family name when they 
marry... What do you think?

"He might, being that Soun has no sons...but it's questionable in that 
he's the only son. If Ranma had a younger brother, yes, for sure as the 
Saotome name would live on as well."

Just thought you lot may like to know...

Kun-chan...