Well, since there seems to be no chance that the bunny could get loose outside and mate, then there is probably no harm in leaving him intact. I have no experience with bunnies, so I don't know if there will be behavioral problems associated with him still having natural urges. I worry about the cat being left intact though, since there is a good chance it could slip outside. I do know that leaving cats intact will result in male spraying/marking in the house, and females coming into heat and yowling. Good thing the kitty is female, because I can assure you, your mom wouldn't want to "bear with" the stench of male cat spray, which ruins anything in your home that it lands on - we had things in our basement that still stunk YEARS after the cat had sprayed on them! Your mom's theory has a few holes - leaving pets in their "natural" state, with the desire to reproduce, but unnaturally restricting them from doing so (by keeping them in a human home) creates stress on the pet. No offense, but ask your mom how she would feel if your dad lived on the other side of a window and she could see him but couldn't ever get near him. Spaying/neutering is one of the consequences of adapting animals to be pets and live with humans instead of their own kind. If we wanted animals to live 100% naturally, then we wouldn't bring them into our homes and make pets of them. Also, domesticated animals who accidentally are set "free" do NOT have the natural defenses that wild animals do, because it has been domesticated out of them, and they will NOT adapt beautifully to suddenly becoming wild.

As far as spaying and neutering humans - I believe in that too! I was "spayed" after having my first and only "litter". Since we can't communicate with animals, how do we know they want to stay intact? Maybe the kitty would want to be spayed, like I did, but can't ask. If people all over the world are homeless and starving, wouldn't it be more humane if they didn't have to reproduce, and try to feed children that they don't have the resources for? Natural isn't always right, because both man and animals have become more civilized and domesticated. That's why man has come up with so many wonderful ways to improve upon nature, and I'm glad for all the miracles of medicine and surgery.