My male will turn into a wimp.
Pets gain weight from a combination of being fed too much and receiving too little exercise.
If you feed your pet a sensible diet and be sure it gets regular exercise, it will almost certainly remain fit and trim. While it's possible for a tiny drop in metabolism to occur after spaying or neutering, this usually occurs naturally anyway as a puppy or kitten reaches maturity. Surgery is such a risk. Surgery is always a risk, but with the huge improvements in anesthesia, testing, and other areas of veterinary medicine, any risks associated with spaying or neutering are minimal. The risk does increase, however, with the animal's age or the presence of disease.