As a guy this is a real thing. The main purpose of the prostate (male g-spot) is to produce seminal fluid. Seminal fluid mixes with sperm and creates se***. The seminal fluid makes it easier for se*** to travel. If the prostate is stimulated it will produce the seminal fluid without mixing with sperm and it will leak out. And since the seminal fluid didn't mix with any sperm it's much more watery and acts as a lubricant. Yeah bl Mangas exaggerate the amount of fluid that can realistically come out but it Is a real thing. Sry for big  explanation  and if I sounded nerdy lol