Another good question would be: how many male crossdressers didn't first get the idea to try female garments on their own? IOW, either someone else dressed them or there was peer pressure to try something on.
As a child, my first time was when my mother had me wear my sister's coat on halloween. There was also a childhood experience of trying on high heels because the kids I was playing with were playing dress up. I believe some CDs don't remember that someone dressed them in something fem as a toddler.
Why some men cross dress and others don't hasn't been studied enough for anyone to really know for sure. We can assume that for most it started at a young age, but why?