Why do girl's always ask me to send them dirty pics but never return the favor..?

Soo, long story short. I've been texting a couple girls, skipped the bullshit and they asked me for pics of my cock. I was like, "sure". Lemme see your p***y, and she's like... "eww! no!" And than I'm sitting here all confused and ****. Sooo, is it something about me, orrrr the way I'm asking or are ALL girls like that..?