What causes an erection?

Patient's question:

Why do I have no feeling when I see a woman's naked body, and my penis does not get erect, even during sexual intercourse (I need continuous stimulation of the penis to get erect, and it quickly becomes flaccid if stimulation stops), and I also ejaculate quickly

Doctor's answer:

Hello: Many men cannot determine whether they suffer from erectile dysfunction, but in fact, they can judge the nature of erectile dysfunction based on the nocturnal penile erection. Normal men of all ages can experience reflexive penile erections during sleep. During adolescence, the average number of nightly erections is about 6, each lasting 20-30 minutes; young and middle-aged adults have an average of about 4 nightly erections; healthy elderly over 65 years old can still have a total of 1-1.5 hours of erection time per night. Even children often experience penile erections during sleep.
Clearly, nocturnal reflexive penile erections are unrelated to sexual stimulation and are not affected by psychological factors. Therefore, they can be used to distinguish whether erectile dysfunction is caused by psychological factors or by medical conditions. If an erectile dysfunction patient has no nocturnal erections or the degree of erection is below the normal range for their age group, it may indicate a medical condition, requiring further investigation to identify the root cause. If an erectile dysfunction patient has normal nocturnal erections, it is unlikely to be caused by a medical condition but rather by psychological factors. There are several methods for self-assessing nocturnal penile erections:
(1) Stamp Test: Use four to surround the penis, stick the overlapping parts together to form a ring, and then go to sleep. In the morning, check whether the stamp has torn. If it has torn, it indicates nocturnal erections. If the overlapping part has come loose, it is not meaningful.
(2) Penile Circumference Measurement: There is a commercially available tape measure (usually paper-based) with one end connected to a square buckle. Before going to bed, wrap it around the penis, and let one end come out through the buckle. First, record the baseline circumference of the penis in its flaccid state, and then