Females that have not been spayed are more likely to get cancer and get pregnant over and over again. Unneutered males wander for miles in search of females so run the risk of getting lost, accidents, fights with other males and being injured which can cause infection. Many shelters are full.
You may be interested in these active petitions
Make the spaying and neutering of pet cats a legal requirement. Kittens should be spayed/neutered no later than four months old.