Scientifically speaking,

Why did mating evolve to involve grimacing and making loud screaming noises?

One would posit that screaming would either alert predators of a tasty meal or fellow tribesmen that someone was in danger.

similarly, we smile when we enjoy something like candy and so why do we look like we are having diarrheal spasms in orgasm?

it just doesn't make sense,
scientifically speaking