Mary could see by the increasingly large bulge surging forward in Mike's slacks that he liked what he saw. And fortunately for him, he was fat enough before the potion that his clothes, unlike hers, were fitting comfortably. The destruction of her pants was as much a relief as it was a seduction attempt. The strain on her waist gone, the cool air conditioning flowing gently on her piggish nethers... it was a thrill. Now if only she could get out of her shirt, which squeezed her hefty torso considerably, especially now that she had more breasts than before.
Mike stood with a grunt, erection standing proudly at attention as he staggered forward on hoofish feet. The arousal seemed to have accelerated his changes, making him even more a hog and even more appealing to Mary's senses. She needed him, in an irrational, animalistic way, and clearly the feeling was mutual.
Instinct pushed her to turn and bend, presenting as a pig would while he fumbled with his pants, his hands less dexterous than before. But he got to work, and removed his pants successfully, dropping them carelessly behind him as he approached the waiting sow-woman. He was more pig than man, but cared not that she wasn't changing at his rate, or even at all anymore. That human female vial had kept her at bay to a degree. An addition Mike did not have.
She felt him fumble a bit behind her, bumping her awkwardly in a frenzied attempt to find his mark, before hitting it right on the fourth try and digging in, thrusting away with a lust that was not there in his human form. The animals needed to mate, were pushed to it by the magic coursing through their systems and changing them in impossible ways. Hormones fluctuated wildly as the transition took place, sending rational, reserved human beings into a mindless animalistic mating frenzy.
And Mary couldn't have loved it more.