Here is my interpretation of how sex works for men with SMA. I am not a doctor, but I took a years worth of science courses in college:
Arousal: when aroused by gentle or rough touch to various parts of the body, sensory nerve cells (receptors) on nipple and genitals send signals to the brain and spinal cord that indicate arousal. When sight, smell or thought cause arousal, the brain starts activating parts of the nervous system to signal sexual arousal. In patients with SMA, the thought parts of the brain are normal- so SMA men can get aroused.
Boners: Arousal activates the parasympathetic system in the spinal cord which sends muscle relaxing chemicals down its fibers into the penis, which dilates blood vessels in the penis and causes an erection. The penis is a smooth muscle and isn't a voluntary muscle, as a result- men with SMA who have limited or zero muscle mobility can still experience arousal and a resulting erect penis.
SMA brain and erection: The brain is not effected by SMA and as a result, imagery, smell, touch, etc can enhance arousal and send signals to the parasympathetic system to maintain an erection- conversely, thoughts of embarrassment, fatigue or anxiety as with an able bodied man can reduce or completely eliminate an erection.
Orgasm/Ejaculation: The sympathetic nervous system gains control over the parasympathetic system and starts the process of achieving an orgasm. The sympathetic nervous system is also located in the spinal cord, and nerve fibers carry chemicals that cause muscles to contract in the pelvic region. The muscles contracting in the penis are primarily involuntary (smooth muscles). When a penis is ejaculating during the orgasm, the smooth muscles contract rhythmically for roughly 3-5 seconds. The involuntary contractions within the smooth muscle of the penis create a wave of pleasure throughout the body and brain and a hydraulic effect that pulses semen up, away and out of the penis- this is the final result of the process of arousal to orgasm. In cases of SMA, with obviously weakened voluntary muscles, the strength of orgasm could potentially be lessened, but it is still possible. To put a simple visual to this: think of a standard penis as a water hose of jizz and an SMA penis as squeezing the lower portion of a toothpaste tube- both can come, but one will shoot further and stronger.