With each thrust, Jeff could feel his body changing. Soon his hands became flipper-like then lost even that much form. Jeff was left with eight long, slender spider legs without any semblance of hands. He could only rub them against Mary's nipples. Although he could no longer hold onto Mary directly, his mass was enough to hold her in place.
Jeff could feel his body shifting further, including his head slowly shifting 90 degrees and his shoulders growing. He still had a human face, but it was reduced to a limited range of motion looking parallel to the ground. He felt his backside slowly swell, bringing his body more in line with that of a spider. Yet he had no desire to stop, finding a strange pleasure in his changes and feeling the uncontrollable urge to finish mating with Mary.
Mary could feel her butt expanding and thought it might be growing into a spider's abdomen. As the feeling of Jeff's hands on her chest changed, she knew he was experiencing further changes of some kind. Even so, this was somehow the best sex of her life. Whatever changes were affecting the two of them, her body screamed for the kind of release only Jeff could bring her. Finally Jeff climaxed, emptying himself into her, and her body shuddered with orgasms that lasted past him finishing in her.
Mary collapsed on the floor, exhausted. Jeff scrambled off of her and she couldn't believe what she saw. Her lover was now seemingly more spider than man.
"Jeff? Can you still understand me? Are you okay?" she asked, concern evident in her voice.
"Yes" he said in a voice that no longer sounded quite the same. "Feel great."
"I...I didn't expect this. I don't know how this has happened, but I think I should get the human vials so we can both change back to normal."
"No" insisted Jeff. "Want to stay like this."
Mary couldn't believe what she was hearing. Then again, what harm could it do? She decided to leave Jeff like this for awhile. Getting to her feet with the aid of her extra arms, Mary found herself unsteady on her feet due to the increased size of her posterior. She made her way slowly to the bedroom to examine herself in the mirror. Her vagina and anus had shifted underneath her bulbous ass / spider abdomen. Other than those not insignificant changes, her only other changes were the ones she already knew about from the beginning of her transformation - extra arms and breasts. Would she change further from more sex with Jeff? Perhaps it was best if she took the human vial now. Jeff could always take the vial later when he changed his mind.