Is it really harmful for body to use it for sex or loss of seman weaken the humanbody?

The body evolved for sexual activity. It is necessary to have sex to continue the species. Sperm and semen are used in that process, and the male body produces millions of sperm, most of which are disposed of if they reach maturity without being used. It will not weaken your body to "lose" semen or sperm. It is there to be used.