{You're hopeless, Mousse.}  The girl tossed back her wavy,
knee-length, purple hair as she looked at the boy with a bored
look. {Just give it up.  You'll never be able to win my hand
through combat.}

One thing that puzzles me about how Amazon customs are normally
presented is that it isn't much of a matriarchy if every single
woman is married to a guy who can beat her in combat either because
he's better, or because she likes him so much that she doesn't
want to hurt him.

In the initial discussion of the Amazon laws dealing with the 
consequences of combat, the rules appeared to apply to outsiders.
The theory was, kill any superior outsider woman so 
she wouldn't strengthen your neighbors with her bloodline and 
skill.  Marry any superior outsider male, perhaps any male who proves 
superior so he would do the same thing for your bloodline and skill.  

Were we ever told that an Amazon CAN'T marry a member of her own village
without combat, just because she MUST marry an outsider male who defeats 
her?  

Could the problem with Mousse be that having challenged her, he is 
thereby legally debarred from consideration as Shampoo's husband, 
(perhaps a law designed to prevent frivolous marital challenges?).

"Wong" has come up with a fairly sensible answer by assuming that 
males in the village who have no exceptional fighting skills are 
simply kept as concubines, but it doesn't really fit what we have seen
of Shampoo in the real series.  
It seems likely to me that under this system, Mousse
would already be Shampoo's concubine and reluctant ally in attempts
to trap Ranma into marriage.  However, of course, in the context
of Ranko and Kaneda at least the depiction isn't internally 
inconsistent, which may be more important than whether it's
consistent with the main universe.  This is, after all, another
universe where Amazons may have different customs.