to imagine my lover impregnating me during sex. Seems natural enough since I've never had children (by choice). There's something magical and powerful about causing conception. It makes me feel very "female" and beautiful.
I used to go into a particular chat room to find men with the same fetish. There were many out there who see it as a respectful, sacred act, i.e. not just a cheap attempt to virtually f*** a girl and get themselves off.
I, like many other women, have gone through life taking great precautions not to become pregnant, so it's special to virtually intend to become pregnant, and novel in a sense. There is a sense of reproductive freedom and freedom from the fear of becoming preggers. That makes it all the more erotic and fun.