A few examples of bad results for pets after mandatory spay-neuter (MSN) has been enacted:
CA:
Los Angeles – first year stats under MSN (more analysis of L.A.’s failure here)
WA:
Comparisons between MSN areas and non-MSN areas in CA:
Lake County has the highest kill rate in the state
Santa Cruz County kills significantly more pets than neighboring counties without MSN
Additional reading:
Why Punitive Legislation Fails (pdf) from No Kill Advocacy Center
MSN Fails to Reach Most Intact Cats (pdf) from Alley Cat Allies