"The spirit is willing, but the flesh is weak", a famous quotation from the Bible, but used with a tongue in the cheek context, it speaks volume to a person who is willing to engage in penetration sex but not able to initiate and sustain an erection sufficiently to sexually satisfy all interested parties.
The causes of the inability to initiate and sufficiently sustain an erection for the purpose of engaging in a sexual activity, be it coitus or masturbation is in medical jargon, coined as "ED" or "Erectile Dysfunction" a.k.a. impotency or the inability to have an erection.
While the causes of ED is complex and can be a mixture of various reasons and causes, it can nevertheless be broadly classified as attributable to:
1. Psychological
2. Physical
3. A Combination of both 1. and 2.
In understanding ED, one has to understand the mechanics behind an erection, the trigger(s) and the sustaining mechanism(s) given that erection is not the default, but the demand mode.
In other words, we are not erected all the time except when we choose not to be so.
Therefore, as erection is an "election" and yet we cannot definitely command the "weapon" to stiffen and perform sexually, there are psychological and not purely physical factor(s)behind the erectile process.
Psychologically, while generally commonly applicable, is nonetheless, subject to individual subjectivity as no two persons will react identically to the same common sexual stimuli.
For example while all full blooded males are turned on by the naked female body, not all males will response identically with a full blown erection.
Barring psychological factors, physically caused ED can be attributed to the physical inability of the penis and the supporting system behind the initiation and sustaining of the erection as specifically intended by the owner of the organ.
The process of initiating and sustaining an erection is multi-faceted and inter-alia involves a sequence that involves nerve impulses from all or any of the 5 body senses to the brain that in turn send out the appropriate impulses and hormones from the brain to the spine, and penis as well as the subsequent response in the muscles, fibrous tissues, veins and arteries in and near the penis, known as "corpora cavernosa".
Among the many common diseases, illnesses, injuries and even substance abuse that can cause this complex physical processes required in the erection process are:
Diabetes
Neurological (brain & nerve)diseases
Kidney diseases
Vascular (heart & blood vessels)diseases
Prostrate Cancer (the treatment to it rather and not the illness per se)
Injury to the penis, spinal cord, pelvis, bladder etc
Surgery performed to threat other illness that can result in ED eg bladder cancer
Venous Leak
Hormonal Imbalance
Prostate Enlargement
Prescription Drugs (over 200 prescription drugs may cause ED)
Substance abuse: alcohol, drugs, tobacco
Generally, the common underlining physical cause of ED is the lack of the sufficient volume, flow and pressure of blood into the corpora cavernosa vessels in the penis that stiffens the penis sufficiently enough for penetration a.k.a. an "erection".
The "treatment" for such a non life threatening problem or "illness" if you like, is diverse and varied but may be classified as psychological, physical and medication.
If the cause is just psychological, counseling helps, however, if the problem is physically, then physical augmentation through physical supports like inserting a supporting length of material to stiffen the penis, vacuum constriction and etc may be applied.
Before the advent of ED drugs like Viagra, Cialis and Levitra, the ED remediation options are limited to psychological counseling and physical augmentation.
These drugs generally aids in the flow and retention of blood in the blood vessels of the penis to sustain an elected erection, as opposed to its default "sleep" mode.
Used under moderation and with proper medical guidance and supervision, ED drugs provide that critical bridge in letting the willing spirit cross over to the other side of attained orgasm, even when the flesh is weak.