Why no Prince Shrek?

When Shrek and Fiona are being presented near the beginning of the movie, they're referred to as "Princess Fiona" and "Sir Shrek".

I'm curious as to why Shrek isn't considered a prince after marrying Fiona. I know he's an ogre, but that doesn't stop Fiona from being a princess. XD

I think most monarchy's are different, and marrying a prince or princess may not make you one as well.... But for most fairytales (and let's face it, Shrek's as much a fairytale as most fictions come) marrying a princess makes one a prince, I think....

Especially considering Shrek was considered a candidate for king.

Thoughts?

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My world is boring; hence the name. :P