[belorn]

Wikipedia has a good hint over the science for ED:

In about 80% of cases, physical causes can be identified.[2] These include cardiovascular disease; diabetes mellitus; neurological problems, such as those following prostatectomy; hypogonadism; and drug side effects. About 10% of cases are psychological impotence, caused by thoughts or feelings;[2] here, there is a strong response to placebo treatment.

If a patient can not get an erection with their partner but can with porn, the cause is unlikely to be physical.