Pet in heat

It is possible. They do have hearts floating around them. Prevent this by getting 'em spayed or neutered. If the Sim doesn't do anything to prevent the mating, the cat or dog may get unexpected litter, from breeding. You can switch the ability in CAS, with the three dots, from "Natural"
to Fixed. If you use a trait (and hidden trait viewer), there is either a "Unspayed/Unneutered" trait, or "Neutered/Spayed" trait. Now if you have the Unspayed or Unneutered traits in this viewer on your pet, replace it with "spayed/Neutered" if you don't want heat or litter.