What Causes Erectile Dysfunction While Having Intercourse?

hi Dr, am a man of 27 years and i do not experiencing having sex with girl for all my age and i was tried one day and my penis do not erect but when it comes to masturbation it erect to the maximum and also when i watch a girl pornography it does the same of not erecting but when i watch male gay sex it erect much. please advise me Dr