Slowly straightening up, Colin rested his hands on Sally's broad hips. As his post-orgasmic bliss started to fade, he startled, his eyes focusing on his girlfriend's VERY changed body in front of him, half in - half out of the tub. Black spots and white fur covered Sally's skin, and a long, ropey tail twitched beside where they were still connected. Rubbing his hands over her rear end, Colin could feel her boney spine and large, pointed hipbones beneath her velvety-soft fur, with much less padding than he'd imagined a cow would have. He couldn't see Sally's face, since it was hanging down in front of her in the tub, but he was beginning to worry about how far the costume's effects would go.
He shifted back, making to pull out to he could better assess the situation, but was surprised by how tightly Sally's body gripped his spent cock. As he applied some force, his softened dick slowly started to slide free of her dripping channel, but Colin's shock only multiplied as he realized something was different about his own body: some of the same white fur covered the skin around his groin, and ... some sort of ... sheath had formed, running from his balls up along his belly until about halfway to his belly button! And his penis! As he continued to step back from Sally, he knew that his human member had never been so long - six inches, seven, nine, twelve... it just kept going! Pulling out of Sally while simultaneously retracting into his new animalistic sheath! As the tip finally slid free, Colin was presented with a shiny, pink, pointed cock that had to be over 20 inches long, flaccid and shrinking back into his changed groin as he watched!
"Holy shit." he muttered, "I... I'm not even wearing any costume parts! Was it the milk? Or..." Colin blushed, one hand clutching at the furry sheath attached to his lower belly. "... would having sex with someone wearing the costume cause this?" He scrabbled in the shopping bag for any sort of warning label or instructions, coming up with the receipt for his purchase. Flipping it over, he noticed some faint writing on the back of the thermal paper:
'ATTN USER: Contact with costume parts and accessories will cause changes to the user's physical makeup, usually for an allotted time specified on each costume item. Complimentary accessories will take on the time parameters of the main costume parts, unless otherwise specified. Contact by secondary parties with costume parts, accessories or fluids produced may cause unexpected changes as well, resolved in the same specified time period. CAUTION: Engaging in sexual activities while wearing costume parts and/or accessories may cause unwanted side-effects to the user and/or sexual partner(s). In the case of pregnancy resulting from sexual intercourse engaged upon whilst wearing costume parts and/or accessories, original time parameters may extend to encompass the gestational period, reverting upon miscarriage or birth.'
"Holy shit." Colin repeated.