Pffffft. My 5yo ds would be quite happy to dress up as a princess. He does quite frequently with the various dress up play stuff we have. He has asked for (and is getting, although he doesn't know it yet obviously) Frozen dolls for Christmas. (and yes, stbx will probably lose grip on reality that ds2 is getting dolls as a gift, but I don't really care - it's what he asked for) But he's also asked for superhero action figures. (I suspect there's going to be an Elsa vs Spiderman showdown, the way I'm seeing it!
)
My dd is 29 now, but as a child she hated pink (still does actually). She was quite happy to dress up as a princess, but in blue thank you very much. Then she went outside and played in the mud with her trucks and fire engines and was hanging upside from the trees in the garden. Not the most princess-like behaviour I suppose, but there you go. 
Personally, I think people sometimes get so hung up on gender stereo typing being offensive that they read far too much into every little thing. Nobody is saying the girls HAVE to be a princess. Nobody is saying the boys CANNOT be a princess. FFS get a grip.
I am not bothered if my dcs want to wear fancy dress as princesses, cowboys, superheroes, nurses, doctors, firemen, princes, pirates... whatever! (although I certainly don't want to shell out a fortune for all those costumes!!)
But why so frothy? What if your dd WANTS to be a princess? Are you going to let her? Or steer her to something that's YOUR idea of appropriate? isn't the whole point to allow them to make their own choices, even if what they choose isn't what you would choose or what you want them to choose?
Girls don't HAVE to be princesses... but there's nothing wrong if they WANT to be one either.