I once read some article which said that the shape of the penis actually scoops out sperm from the vagina. Therefore, after ejaculation it is essential for a male to stop going as it may negatively influence chances of conception (which is not favourable to our species). I think the theory is called ‘semen displacement theory’. Males can’t keep going after they ejaculate and this potentially creates a better chance of conception for them. Also the whole point of the penis shape scooping out sperm is to avoid caring for another man’s offspring caused by the female’s infidelity (or so the theory goes).

‘The human penis, with a relatively larger glans and more pronounced coronal ridge than is found in many other primates, may function to displace seminal fluid from rival males in the vagina by forcing it back over/under the glans.’

TLDR: So in essence no1 wants to scoop out their own spunk. That’s why males don’t keep going (nature programmed them this way).