What happens is when we masturbate to porn too much our brains become wired to that. Think of like this... when you were very young and never had access to porn you could masturbate without any visual simulation. As you got older and masturbated to porn your brain needed some visual simulation to get a lot of pleasure from it. Same with real life women and porn.
Another factor is masturbating using the death grip. Your private parts become accustomed to this tight grip and when you have sex with a real woman you go soft because your dong prefers something tighter.
I used to have this problem a lot on punts till I cut out the death grip and porn.