A man can still get hard if he is paralyzed from the waist down.
However the man getting hard or having an erection when being paralyzed from the waist down is often a reflex erection which means it is often a result of physical stimulation instead of psychological arousal.
When a man is paralyzed from the waist down the nerves that are actually responsible for psychogenic erections can be damaged by a spinal cord injury.
This means that the paralyzed man may not be able to full maintain or control their erection during sexual intercourse.
Some men that are paralyzed from the waist down may also notice they have an erection when cleaning their genitals, pulling clothes or blankets over their body or when changing a catheter.
This type of stimulation to the mans penis may produce an erection even if it was not intended.
Even reflex erections can be sometimes happen at times that are not convenient or even be brief.
If a man that is paralyzed from the waist down is having trouble with erections some erectile dysfunction pills can help such as Viagra.
Viagra is one of the easiest interventions and most common oral medications that can be taken by men with a variety of spinal cord injuries to get an erection.