Because it is "naughty" and it is fun. There's a thrill for men like me to play dress-up and when you are in that mode it's fun to fantasize about being "taken." You don't have to be gay to enjoy playing with yourself "back there."
Your more than likely someone who is both 100% comfortable with their sexuality but more importantly who and what they are. Meaning that you probably more open to “anything that happens tween a dude and a chick is AWESOME!!! Right??? So what’s the fear just make sure that you sig. other is ok with it and enjoy!!!
No worries, same fetish here. Every 2-3 months i get this urge to have some CD fun. After the deed i'm a bit depressed and think its gross but it passes. Knowing other heterosexual man do it too helps a lot. Cheers.