Cats and dogs are not able to mate and interbreed. In order for two different species of animal to reproduce, they have to be closely related enough that they share most of the same genetic makeup.
For example, it’s common knowledge that horses and donkeys can interbreed to produce mules, though the offspring mules are infertile hybrids and cannot reproduce. However, horses and donkeys, while different species, are both part of the same genus, equine. Their similar genetic makeup is what allows them to mate.
On the other hand, cats and dogs do not share enough genetic material to produce viable offspring. A sperm cell from one species would never be able to fertilize an egg from the other.