Through clever maneuvering.

She immediately moves and tries to stuff her cock into your exposed crack on the top of the low-riding shorts. She only manages a few inches since the tight material is actually wedged into your crack. Next she tries stuffing it in the bottom of the right leg. The legs are very short, so it is the shortest route to your hole. She has you bend over and when you do, a fold opens on the inner back side...and in she goes. Your right buttock is pushed aside as the dick moves the short distance to its goal. She does it. You feel the monster cock move inside of you fully. Then she fucks you. It feels interesting with your ass shaped by the denim. The dick itself is wonderful too. She pulls on the tops of the shorts so it wedges further up your crack. You could swear you can feel the bottoms of your cheeks pop out from under the shorts as she pulls them up. Reaching around back, you can feel the bottom curve of your ass sticking out. She puts a hand over yours and guides you around your right buttock. It really does feel like a woman's ass; you have grabbed enough in your life to know. As you had suspected, the shorts have shaped your ass with the tight material. Liz is fucking her first female butt...sort of. At least she has had the opportunity if she doesn't get to do a real one from behind. You are thinking she is thinking the same thing due to all of the dirty talk she has been using. Terms like 'bitch', 'bubblebutt', 'heart-shaped ass', 'anal whore', and 'slut' come from her mouth. Well, guys can be sluts too, they are just more often called 'players'.
"Almost done, Liz?" you ask after a while.
"No way! I'm gonna enjoy this fully while I can!"
"Well, I'll dress up for you any time, of course! You can save some for later!"
"Oh! I didn't even consider you would do it all the time for me! Here, let me finish up before I wear you out!" Liz gives it a few more thrusts then pulls out.
"I didn't mean all the time..." you try to tell her, but she isn't paying any attention to your voice now. You know she is cumming from the high-pitched moans but you don't feel anything. Then you feel drips on the exposed bottom cheeks off your butt. She came on your girly-shaped ass with the shorts still on! You slowly feel the wetness penetrate the denim as drips run down the backs of your thighs. She gives a sigh as she flops down on the sofa. "A work of art," Liz breathes. You raise your hands and feel your slimy denim-encased cum-coated ass. You smear around the ropes so your molded ass shines for a bit until it all soaks in.
"And look, I'm ready again," you tell her as you turn to show her your hard cock poking out of the front of the ruined shorts.