There is such a thing as a naturally occurring "gaper". If the model is comfortable and relaxes her butt muscles the right way. I've seen it happen before and the girl didn't have a giant dick up there a second ago. It's an enticing shot/view though.
There's one of Sonya that I've always wondered about, it's from set 227, originally I thought it was airbrushed but as I went to post it I looked again: