Masturbation is perfectly normal, and in many ways, it's encouraged to explore your own body and find what works and what doesn't. This way you'll know how to be more comfortable with yourself, your body, and know what to ask of your partner when it comes to sex. So many institutions frown on this activity, and some outright condemn it. As a result we feel remorse, guilt and shame over it. I find that shaming a way to fulfill a fundamental need we have is just medieval and barbaric. As a result, many are absolutely clueless when it comes to their bodies and sex and can't fully enjoy the experience.

That is to say, there is such a thing as become downright obsessed with the activity. It does release endorphins into the body and brain and this release can be addictive in a way. That and going overboard can cause physical strain on the body. Our bodies can only take so much of a physical activity at a time.

Yes, masturbation is a physical activity. The heart rate increases, the body releases many hormones including the aforementioned endorphins, and the physical reaction of the climax can leave you feeling both breathless and physically tired, to a degree.

Anyways, my point is, sex and masturbation are perfectly natural and shouldn't be frowned or looked down upon. However, on the flipside, if a person doesn't want to participate in the activity they shouldn't feel they should do so out of obligation or because everyone else is doing it so they should too.

I hope I helped clear things up for you.