It's a woman's changing room, not a whoever-you-like-seeing changing room. There is no reason (other than some sort of puritanism) to get freaked out about girls seeing penises. In this context, it's just a body part.
This is totally asinine. There are a many reasons why... With just a little thought I'm sure you can think of at least one.
Sending unsolicited dick picks is sexual harassment. A woman changing in the women's changing room isn't.