Hmm...I don't know. I have a little 5 year old boy in my resource room class, and he's fixated on everything related to women. He can't draw anything but little dolls, with long hair, and he thinks that a red marker looks like lipstick. Plus he has a fixation for long hair. I have a puppet in the classroom that has long hair, and whenever the kids get free time, he wants to play with that puppet, and he likes to brush the hair. At the beginning of the school year, he was a real behavior problem, and once his regular classroom teacher figured out he had a fixation for feminine things, she brought a doll with long hair to school and told him if he can behave, he can play with the doll. That boy has behaved wonderfully in her class ever since.
Scary.
To answer the question, if I have a little girl AND a little boy, and he wants to play with her dolls, I would allow it, for a short time. He will NOT be trying on Mommy's lipstick, shoes, makeup, etc. I am NOT trying to have my baby end up like Prince when he gets older.