It all happened so quickly. Neither Mary nor Rob were fully aware of their actions, as first Mary presented herself and then Rob took advantage. It was almost like a dream as they rutted, each rhythmic swaying motion punctuated by inhuman grunts and bellows.
Mary could feel the changes as intercourse caused another mutation. She could feel herself becoming heavier, more massive, with a larger, more appropriately bovine frame. She could feel Rob's weight increasing upon her back. It forced her down onto all four hooves, her bovine transformation going well past the halfway point and perhaps more towards completion.
Rob had lost himself to it completely. He wasn't thinking in human terms. He was a bull in rut and that was all. He hadn't had an appropriate human serum like Mary had to mitigate the effects, so as he continued to thrust against the cow beneath him, he changed more and more, muscle adding to his widening frame, face extending into a bull's muzzle.
Finally spent, the bull climbed down off of the cow's back, glancing around the room. He was hungry now, and they were not in a field to simply graze upon.
Mary followed the bull's gaze towards the glass back door, and a moment of panic shot through her mind. She hoped to God that he wasn't thinking of doing what she thought he was thinking of doing...