Very ingenious idea.

Note, birth control pet medicine would probably require lots of research/lab testing to identify any potential side effects. Would PETA be upset with the testing of animals medicine on animals?

If you keep your pet indoors, there is no problem. Likewise, a leash works well when taking a walk.

Cats want to go outside, and most rural and suburban owners allow them to do so. They don't cotton to leashes.

The problem with pills is that problems remain. Female cats will still come into heat and howl for weeks (especially if they're Siamese), and male cats will spray around the house and roam widely outdoors, looking for rivals and hookups and getting into trouble.

Still, many people don't want to pay for an operation, so this would be a good second choice.