This is what Ron White calls “the lazy dog.” 
Even that’s not quite it. It’s more that people just slept around a lot before marriage, and whenever a girl got pregnant, she had to marry whomever it was that she was sleeping with at the moment. It was taken for granted that young folks “courting” each other would be sleeping in the same bed.
I know I have.
Hello? m/f?
Best use of emoticons I’ve ever seen.
Assuming that is true (presumably you are talking about prehistoric times), what reasons do you have for thinking that they had the same sorts of problems in figuring out how to have sex back then that many people have now, when we wear clothes?
The non-joke answer is that most Puritan congregations became Congregational Churches in the 18th century. But the Puritan-Congregationalists became far outnumbered by the ordinary Church of England and Presbyterian immigrants who settled in New England in the late 17th and 18th centuries.
It’s worth mentioning that the earliest experience for most boys and girls is with Rosy Palm and her Five Sisters. No one needs instructions on doing that. People figure it out on their own.
That experience should help with the first sexual encounter. The boy knows what works for him. The girl knows what she likes. They share information. 
Yes, I think it is instinctive. At least that applies to the kind that lasts less than 5 minutes.
Extended sex that allows women to orgasm takes study and a guy that is willing to learn.
I thought I had invented it. I was ready to write a book.
There is actually an example of that which produced offspring, they say.