It wasn't his idea...she was the adventurous one, the one who always suggested doing some urban spelunking, checking out abandoned buildings...and usually getting railed in them. He wasn't gonna complain that she had a bit of a freaky side, sort of exhibitionist without any actual risk of being seen.
Something felt different about this house, though. Even though it was on the outskirts of town, the remnants of an overgrown pasture could be seen behind it. This wasn't that unusual, there were a few farms at the edge of the city...but there was just an odd sense of foreboding about the place. He could tell that only made it more enticing to her, as she ribbed him for being afraid of a junky old shack.
They wandered around, showing each other different trinkets, each trying to ignore the tingling across their bodies. Finally he heard the low moan that signified she was probably ready to get down to their usual business, and turned to find her nude and making eyes at him. The fact that her nose had ballooned into a broad snout didn't even register...his own nostrils flared and expanded to take in her scent. She smelled so good, so ready. He got into position, gripping her furry flanks...as he pushed in, he was dimly aware that something wasn't quite right. She felt deeper, but at the same time he felt longer, fitting her strange anatomy like a glove. She mooed as he bottomed out, and her changing hands clacked on the floor as he thrust. Their bodies warped and shifted as their frantic session continued, and neither would exit the same as when they'd entered...