In this circumstance I would do anything but knock her out.
He knows the laws are crazy already, he was just plain stupid to KO her.

	In case you've forgotten he was trying to disarm her, and puerly by 
accident Shampoo got knocked out by her own Bonbori.

He may not have expected to have her try to marry him but he did expect to
have her after him.  It's an unfortunate circumstance for Ranma but his
own lack of care got him into it.  He could have run off with Akane, or
just blocked Shampoo from Akane, or given Shampoo a bogus story, or
changed himself and lead her off, or... shall I go on?

	If he'd successfully disarmed her he probably wonld have done something 
like this, however as you may recall, his block knocked the head of Shampoo's 
weapon into the air, and it just happened to land on her head.  Ranma had no 
intention to do anything beside stoping her attack on Akane, his "defeat" of her 
was unintentional and accidental.