Anal is best...

She lets you position your cock yourself this time, and you press the head carefully against her arsehole. She's clenched tight, and there doesn't seem to be any give at first, but then you kiss her neck and tell her to breathe, that it will all be alright, that she's in for the treat of her life, and slowly your cock seems to find purchase, her tight arsehole relaxing enough for you to push ahead, head full of her gasping breaths and the sight of your cock slowly, so very slowly, disappearing into her hot, tight arse.

It takes time to get your full length in, and she moans and whimpers but then you're there, and it blows your mind, and the little girl who hasn't said a word to you all evening is quivering around you, beneath you, and your hands on her hips are shaking as much as she is. 

She give you the go-ahead and you pull out two inches, then three, rocking back and forth as she slowly relaxes enough for you to get into a rhythm. Slowly her whimpers fade to mumbles, then to moans, and finally to cries of delight as you push into her a little harder, a little faster, her tight little arse welcoming you with each thrust. Soon you're thrusting into her with abandon, grunting at each move, glorying in the sounds you can make she makes. You're glad you came before, glad it's given you some staying power, and the feeling of heady abandon fills your head as surely as your cock fills her pale body, stark against the dark sheets. Then she bends low again, her face pressed to the mattress, muffling her shouts as you pound into her in earnest. You're staring down at the glory of it, the heart-shape of her arse with your cock in the middle of it, stretching her tight and filling her to bursting.