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Semen is not a precious one, just like any other secretion of our body. Likewise masturbation
also a normal and safe activity.
If you got marry at the age of 18, what would you do? Intercourse with your wife. Nobody will tell that sex is harmful. If not married we cannot gratify our sexual instinct with unknown ladies as it is not allowed and also lead to sexual transmitted diseases. So what everybody can do, is only masturbation. If sex is not harmful the masturbation is also not harmful.
If you abstain from all type of sexual activity then the semen produced in the body has to come out if the store reserve is full. It is natural.
It is because of your concern and worry your erections are less rigid. When you relax and touche by a girl your erection will be normal. Nothing to worry. Everything you have are normal only.