If you want your gf to let you fuck her in the backside you gotta make her want it as bad as you do. I started fooling around with my gal's butthole and she kinda enjoyed it cuz it was something fresh and fresh after a year or so of simple oral and vaginal sex. I wanted to be adorable and stuff, but once she got used to my cock being inside of her backside she virtually prayed me to fuck her harder.