Myth: Neutering male cats causes urethral obstructions:

§ Fact: Exhaustive studies have indicated that obstructions are not affected by whether or not a cat is neutered. In fact, neutering diminishes the likelihood of prostate and testicular cancers and perineal hernias later in life. To prevent urethral obstructions, make sure your pet is eating the best diet possible.

(taken from http://www.nccn.net/~paleague/myths.htm )

Tell her it is a very quick and easy surgery for the boys and they bounce back very quickly. One day of discomfort makes up for a lifetime of happiness.