If you want your girlfriend to let you fuck her in the ass you gotta make her want it as bad as you do. I started fooling around with my gal's butthole and she kinda liked it cuz it was something new and fresh after a year or so of ordinary oral and vaginal sex. I wanted to be nice and stuff, but once she got used to my cock being inside of her ass she virtually begged me to fuck her harder. I had the most intense orgasm ever and so did she, and now going backdoor is our favorite thing to do.