It was like having sex with a completely different person for Guy. He had always been used to making love to Stacy as a man of normal size with a dick of normal size. Being a 600 lb muscle god with a shaft to match changed the parameters. For one he could no longer be on top, or else he'd crush his girl to death! He fell onto the bed with a loud thump and saw Stacy literally climb on top of him. She sat on his thighs and removed his tight underwear, revealing his giant new snake.
"Everything's grown so much, Guy!" she said and grabbed it. It was bigger than her arm as he got hard and she got more and more turned on. "Why is your skin so greasy?"
"Must be the ointment that turned me into this. Maybe there's some left -- Oooh!"
She began to play with his dick and every giant muscle in his body convulsed to the pleasure. He groaned and closed his eyes, feeling only pleasure as Stacy licked his dick. "Oh god Guy!"
"Don't stop!" he yelled and his voice was almost unfamiliar to him!
"You're growing! You're changing!"
"Baby don't stop!" He opened his eyes and Stacy looked at him, afraid. "Don't stop!"
Guy continued to grow as Stacy looked at him. She seemed scared at first but then as her man became even bigger and stronger, she went with it and started to stroke his giant dick with both hands. Guy growled and felt his body sliding off the bed, growing larger and larger. "Oh my god you're such a beast!"
The bed broke beneath him as he grew too large and too heavy. He opened his eyes again to see his giant dick standing up straight. She could use it to pole dance!
But something else was different. He looked at his massive hands to see that they were growing thicker and hairier! "What is this?!" he thought. "What is the cream doing to me!?"
The pleasure was too much. He sat up and grabbed Stacy with one hand, placing the other one on the wall and clawing at it. His hands had grown claws he had just dug deep into the wall. Stacy started to laugh and he could feel how wet she was getting. He grabbed her and slowly placed the tip of his cock in her pussy. She yelled in pleasure. "Keep going! Change more!"
Guy was beginning to lose control as he transformed into something far from human. His muscles and bones kept expanding and he was growing a thick mat of fur. His face was getting animalistic qualities, like yellow eyes and great, pointy tusks. He began to roar as he finally slid inside her. He had transformed into a giant beast and Stacy was almost too happy about it. Guy roared again and they came at the same time. The massive hairy monster fell back and broke the bed further as Stacy fainted from the pleasure.
Both woke up an hour later.