The Pet Issue

My 18- year old stepson wanted to adopt a stray cat that his gf found in their place. He said the cat is very nice and it won't scratch anybody. We have a pet rabbit that he adopted and it lived in his bedroom centuries ago. Now he wants a cat and he said he will bring the rabbit out to the family room again so the kids can play with it. I always wanted to have a dog but hubby said no. Would it be fair to adopt a cat and not a dog? I firmly told hubby that I don't want the cat around since he never said yes to a pet dog. Secondly we are selling the house soon since we are moving out and I am certain that it is hard to sell a house with pets. The cat will stay in his room he promised. The again we said no period. He wanted to know a good reason why we said no. His dad told him to find his own place since he is of age to go on his own. This way he can live with his pet cat. End of story.