my theory:
 
The Amazon Solution
 
After Ranma rejected Shampoo under the influence of the truth drug, she
knew that they would never wed. In order to maintain her honor despite
returning home without her "airen" she used a drug/pressure point/magic
item to make Ranma consummate the Tendo engagement with Soun. If Ranma had married Akane, she would have been merely an obstacle to kill; but by "proving" his interest in a man, Ranma freed Shampoo from any stain on her honor (Amazon marriages are, after all, designed to favor the woman [as per Ranma's observation in the most recent chapter]).
 
Afterwards, Mousse got his personal revenge on his hated enemy by putting Soun under a hypnotic suggestion to "attend to his wife's needs". Soun's excessive drinking was a result of guilt over actions and desires whose origins he didn't understand.
 
-Essex