How To Prevent Semen Leakage?

I am 25 year old and when I watch porn or just started loving my partner ...i feel like my sperm is coming out and it comes out automatically within five minutes without masterbating or doing intercourse. I thinks too much about romance and love making . Now I am in very much tention about this. This thing has started from last month. So please answer me what is this happening and what should in do to prevent it and becomes a normal person.