I would love to see a Spay/Neuter hack, as well as one that makes pets randomly mate. When I found out that you had to tell your pets to get it on, it annoyed me. It's bad enough that your Sims cheer when the animals mate, but then they have to tell them to do it, too? Why do I get the feeling that someone at EA is twisted in the head?
My grandmother has a poodle that tries to hump you whenever you curl up in a blanket. When I was little I would try and drag my blanket around the house, and that dog would be following right behind me, trying to assume the position and molest my blanket. I thought it was the funniest damn thing. Now, not so much.
I also have three cats that are impulsive sprayers, and only two of them are male. Only one of the boys isn't fixed, and the little girl never sprayed until the boys started spraying. Quite irritating, actually. Spray is hard to get up effectively, and whenever it rains, you know for sure if you missed anything.