Cats are less likely to have breast tumors, but if they do have them they are almost always malignant. Spaying almost eliminates this risk. Uterine disease is no longer a problem after spaying. Ovarian systs that can be sometimes very painful are no longer even a consideration after spaying. Neutering significantly lowers the risk of prostate cancer in male dogs.
Neutering eliminates the risk of testicular tumors as well.
Spaying and neutering also prolongs a pets life. Almost by twice the life span in cats, and a significant number of years for dogs. It cost too much. It will also cost less than the vet bills incurred from your male running after the female in heat up the road, and getting injured by car. There are programs for assistance for those who cannot afford to get the procedure done.