In my experience, most girls love to have their ass licked and played with while getting fucked. Start slow and get her comfortable. Always lick her ass when your performing oral, go back and forth, lick her ass and rub her clit at the same time. Touch around her asshole when you’re in doggy or cowgirl and when she’s comfortable with it slide the tip of your finger in. Girls cum a lot with the extra stimulation of a finger in the booty! Once they realize how good it feels they will want to feel what it’s like to get fucked in the ass. Most girls absolutely love it but there will be some that consider that exit only!