Answer and Explanation:
A marriage between Cathy and Heathcliff would have been explosive and unstable. Both characters are passionate in their attachment to each other. But passion, unlike real love, is not meant to build a relationship. It destroys, demands, imposes, prohibits, entices, etc. Passion is strong, that is for sure, but so are its disadvantages. Cathy, who grows up wealthy and spoiled, is used to mistreating Heathcliff instead of treating him with respect and expressing her feelings for him in a healthy way. Heathcliff, hurt, proud, and wild, ends up doing the same. He often displays behaviors that show he feels entitled to Cathy. If they were to marry, their relationship would most likely be based on jealousy, anger, and resentment.