Why does my penis only get erect during masturbation?

Take a deep breath; this is quite a typical phenomenon and there is no need to freak out. When the sperm does not ejaculate for a while, it may be ejected by the body automatically, even in the lavatory. This is because the old sperm becomes obsolete and your body needs to get rid of them. Don't worry, it's all too normal. Just keep the genitals clean and maybe masturbate from time to time so as not to experience this.

Chances are very less since sperm on hand cannot get any female pregnant. 

You may have penile nerve irritation. The symptoms include a sudden sharp feeling of yourself during the night. Doctors may advise the following: take a break for a while, do not do anything that hurts you, and visit a doctor’s office and check on this matter with professionals.

Some normal things may happen like fe­ar, worry, and body hurt. This is okay since it is new. Go slow, talk openly with your boyfrie­nd, and make sure you both want it. Use prote­ction to not get pregnant and STIs. Take your time and only do what you both agree­ is good. Your health and safety matter most he­re.

Your age means that it’s completely normal for guys your age to have wet dreams. It happens when extra fluids are released by the body during sleep Sometimes this is caused due to sexual thoughts, or because there wasn't enough time before going to bed for all necessary liquid releases. You should relax before going to sleep and avoid any stimulating activities so as not increase the likelihood of having a wet dream It needn't be worried about, as this occurs naturally!