On Sun, 15 Sep 1996, Caroline Ann Seawright wrote:

Ranma thinks of Ukyou as a guy. He's thought that for
over ten years - that way of thinking just won't suddenly change over
night. If Ranma was to marry Ukyou, HE'D have to be homo. He's clearly
not, and is too macho to even think that way.

How much does gender matter to a six year old?  Ranma thought of Ukyou as his
friend; knowing Ukyou's gender would make no difference in his thoughts about
her.
By the time Ranma got old enough to notice girls, it would have been years
since he had last seen Ukyou.

And, since he's always THOUGHT of Ukyou as a guy, he's not about to stop
just because he sees she's female. Would you marry a girl who used to be
a male, ten years ago?

Well quite frankly, when you are 6 years old, the sex of your friends
hardly is an issue.  At that age there are no visible physical differences
or behavioral differences, and it just doesn't matter. 

It becomes quite obvious that when Ukyou reappears, Ranma starts
to think of her as a girl.   However, he does think of her as
his old friend first and foremost, and as a fiancee second. 

Once you get to the okonomiyaki sauce episode, you do see Ranma
starting to take her seriously as a marriage prospect.

I'd imagine that had Akane not been around to smack Ranma over
the head repeatedly during that story arc, Ranma very well
could have ended up marrying Ukyou.  Just look at his reaction
to when Ukyou cooked for him etc. -- he was completely malleable
and manipulable :)