When my son Danny was in preschool and decided he wanted to be a princess for Halloween, I didn’t know how much I would change. I grew up in the 1960s in a mixed race family that embraced difference. I marched to end war and racism and to support immigrant rights. Yet, being Danny’s mom, I discovered I had deeply held prejudices about gender. In the beginning, I wondered if there was something wrong with him. I tolerated his...