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. 

If he does, why isn't he running away from her like all other girls?
Because he thinks of her as a friend, not a girl.

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

Seriously, like trying his hardest to get RID of her.

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 :)

Well, each time, it was because she offered him free food. If he
was going to marry someone for free food, he would have married
Shampoo a LONG time ago - free food doesn't equal marriage to Ranma.

Kun-chan...