Is prone masturbation a harmful/wrong/abnormal way of masturbation? does it really affect someone's sexual life in future? thank you.

Not wrong or abnl: So long os one is not creating physical harm, there's no worry.

Prone masturbation: Frequent male prone masturbation places significant pressure on penis & nerves at its base & may cause numbing to arousal / pleasure with other positions. Seek help for delayed orgasms, erectile dysfunction or inability to achieve orgasm in other ways. It may lead to back pain in males & females. REF: https://www.healthline.com/health/healthy-sex/prone-masturbation#effect-on-sensation