1.
Is it normal that an erection falls as soon as penetration or manual stimulation stops?
Do most men function in this way?

2.
During the sexual excitement,I (or my girlfriend) almost always have to additionally
stimulate the penis to achieve an erection.To be more exact,the prostate secretes the
fluid normally during the excitement,and I am really turned on.But to have the erection,
I have to manually stimulate the penis.This happens during masturbation while watching
porn-movies and also during a sexual intercourse.
Is this normal,because,since my earliest youth I have been expiriencing this problem?
What could be the cause of this problem,and what would be posible ways of treating it?
I also have to add that my nocturnal and reflex erections are normal,but the psychogenic
ones are not-in spite of the stimuli.
I am 28 years old,in great health and an active athlete.My blod and urin results are OK.

Thanks for any help!
Adam