B.C.
I think it's a development stage.
Most go through it one way or another, some sooner, some later.
When my son was in pre-school he thought girls were yucky.
When I pointed out that Mommy is a girl he got this horrified look on his face and proclaimed "Mommy is NOT a girl! Mommy is MOMMY!".
It was hard not to laugh but I was SO glad he didn't think I was yucky.