What is wrong with prone masturbating? why is it dangerous?

Prone masturbation: There is nothing wrong with prone masturbation. It isn't dangerous and does not cause sexual dysfunction. More about prone masturbation from Kinsey Institute (Institute that specializes in sexuality) : http://www.idsnews.com/article/2015/10/prone-masturbation-a-threat-probably-not

Prone masturbation: Frequent male prone masturbation can put significant impact on the penis & nerves at the base of the penis. There may be some numbing to arousal / pleasure. There are also some reports that frequent prone masturbation may lead to back pain in both males and females. REF: https://www.healthline.com/health/healthy-sex/prone-masturbation#effect-on-sensation