Other mistake: At the end after the barn fire, Marcia is at a tavern, ordering a rare steak. While it is believable that she escaped the barn fire, it is unbelievable that she is in the tavern wearing the same dress and necklace she wore prior to her transformation in the barn. All of her attire would have been rended in the transformation and lost in the fire. Unless her wardrobe consists of duplicates of the same clothing.
Revealing mistake: The final shot of the sex-scene between Bill and Marsha is very obviously a piece of (rather cheap-looking) 2D cell animation. The actors look like cartoons, the fire is simply the same few frames repeated over and over and the background looks obviously painted.
Answer: Whether it be dog or wolf, the primal instincts take over. Every creature, in nature, has an urge to seek out their own kind. When the husbands were bitten, they are consumed with the desire to hunt, kill and mate.