A question posed to another blog:
That's a disappointing answer. Well, a number of reasons could be causing you to lose your erection. It depends on the reference. Is this during sex with another person, or when you try to masturbate? Are you you able to cum before you lose it?
Do you wake up with even a slight boner? If you can't stay hard until you cum under any circumstances, it's not normal and you should see your doctor.
If it's when you're engaged in activity with someone and you have difficulty keeping a boner, but you can when you jack off then it might be that you aren't enjoying the sex.