The erection is a highly sensitive, cascade-like process which I would like to explain here very simplified.
First, the blood flows through the pelvic arteries (A. pudendae) into the penis arteries. Via venules, the blood leaves the penis again.
If the penis is slack, the blood flows past the so-called corpora cavernosa of the penis almost completely, because the small muscles of the corpuscles are contracted and prevent the inflow of the blood into the corpus cavernosum.
Through sexual stimuli that come from the brain, the small muscles in the cavernous bodies are relaxed. The blood flows in, the discharge from the penis is blocked. The cavernous bodies fill with blood. The penis becomes stiff and firm.
This process is mediated via neurotransmitters (NO nitric oxide) via the nerves of the penis. By the way, the potency agents such as Viagra, Cialis, Levitra etc.
A sexual stimulus acts on the brain, this sends impulses to the nerves in the genital area,
In this case, NO is released, which then becomes the relaxation of the muscles of the corpuscles and (2) to block the venous drain from the corpuscles.
Relaxation leads to the erection not stress or tension!
This could also be an explanation why it usually comes half morning to the erection. Here the man is still relatively relaxed, the testosterone levels are quite high and the stress low.
Ergo: Sex succeeds well in relaxation and bad at stress!
The more often the man relaxes e.g. Massage the better the sex!
Ejaculation and orgasm
The ejaculate is emitted by the rhythmic contraction (contraction) of the vas deferensor and the prostate and the ejection through the urethra. A backflow of the ejaculate is not possible. The orgasm is associated with the ejaculation and ejaculation a pleasure, relaxing sensation with many hormonal effects. The orgasm is mostly limited to a few seconds and leads to the reduction of the sexual excitement but not necessarily also to the slackening of the penis.
If sexual stimulation continues, the erection can be maintained and further orgasms are possible.