Originally Posted by HD Hockey Guy
You are applying a more modern meaning of the word than it's old world origins. Until she is indeed a queen to Joffrey, this description fits for many reasons.http://en.wikipedia.org/wiki/Courtesan
We're modern people talking to each other, so I tend to think that when someone uses a word, they are using it in the modern sense unless the context reads otherwise. The context in the original post read to me as if it meant "she plays the game of court well" which made me think the intended word was courtier.
Not lost here is that Margaery is indeed looking to trade on her sexuality which does brings up some of the more modern tones.
It really kind of doesn't. Acting seductively with her very difficult fiancee does not make her the equivalent of a woman who sells sex for money. We would not call a male character who we've never seen have sex with anyone a man-whore or a gigolo. Referring to a female character with a synonym for prostitute is no different. If you want to call her manipulative, cunning, sly, a game-player, or a liar, all of those are to some degree accurate. Calling her a prostitute is not.
One of the overriding themes of this show is how those without obvious power come to exercise it in their own way. We've seen actual prostitutes who do accrue some degree of power -- Ros, through Littlefinger, and Shae, through her access to Tyrion. (See, I have no problem with calling an actual prostitute a prostitute.)
For a noble born woman, presenting herself as a desirable marriage prospect and then becoming the wife of a powerful man is one of the only ways to achieve any degree of power and security for herself. The decision about who the husband is to be is usually made without her input at all, and nobody particularly cares if the woman (girl) will be happy or not -- the decision is made to benefit the family, not the woman. This has been the situation with nearly all the noble marriages we've seen on the show to date -- Ned and Catelyn (and Catelyn was originally engaged to Ned's older brother), Dany and Drogo, Cersei and Robert, Lysa and Jon Arryn, Marcella's engagement to a Dornish prince she's never met, apparently Olenna Tyrell's marriage as she implied last night, and Robb's now-broken engagement to a Frey to be named later.
In that context, Margaery is no more a "courtesan" than Sansa is, who, in her younger and far less sophisticated way, was doing the same thing in season 1 that Margaery is doing now -- trying to please her future husband for the benefit of her family.