If the assumption that the penis becomes weak after masturbation was true, we would have a world full of people with a so-called weak penis. The penis is designed to perform the function of sex and urination. Masturbation is an absolutely safe way for the release of pent-up energy.There is no risk of any sickness or pregnancy.
In fact, it is a pointer to normal sexual development. It is like the body preparing itself for the ultimate test of penetrative sex. People with sex problems are encouraged to fantasise and find out their sex points. The only effect of masturbation is releasing the semen. The frequency of masturbation may be more in the teenage years, due to the novelty factor, but it may slowly decrease. Couples engage in mutual masturbation from time to time, even after marriage.