How do they know how to have sex? Plothole?

Although, typically, historically, the mother would talk to the daughter about things, and the father their son, so it wasn't as though they were going in blind, and remember, this used to be normal until the 1960's, even if people weren't always versions when they got married. I waited till I was was married, but I was not naive to what sex was for various reasons. Whereas, I had some friends who also waited until they were married, didn't ever watch porn or anything like that and ended up not really knowing how things worked cause no one ever explained it to them either, but to my understanding, they figured it out eventually. Also, I wouldn't say that is typical in reality, even in couples who wait until they are married.

Also, when you are married to someone and committed to them, there is plenty of time to learn, especially say 100 years ago (or before) when we had more free time (which seems ironic since we used to as a world, cook everything from scratch and build everything by hand, but things were not rushed). Typically though, I think their romance would have been even more clumsy than it was in the film if this were reality since no one knows anything about sex at all seemingly, but I felt the emotions were extremely well portrayed in the film, the anticipation, the longing, etc. The need to be with people, it is innate in our nature, we all need companionship, whether romantic or friendship,

When did prairie dogs start having sex with chickens? I musta missed that Attenborough documentary.

I beg to differ, but the only instinct of a human is to suckle. Everything else, including sex, is learned!

Doggy porn!   

Fanboy : a person who does not think while watching.

Humans are one thing, animals are another thing. Many things animals do by instinct, humans must learn by watching other humans doing that and copying them. For example, animals can walk by instinct but human babies must be taught to walk upright, otherwise they will walk on their arms and knees/feet their whole life (so it seems that walking on four legs is instinctual).

The movie's premises are that they were never allowed to touch other human beings or watch other people touching/having sex.

Maybe they were taught about human reproduction at the anatomy class in school, so they got the idea about sex. Still, touching their mouths in a sex ritual named kissing is not taught in anatomy class and it's not instinctual. I really don't think that cavemen kissed each other.