And animals can give birth without help too.
Humans on the other hand... if I had given birth I wouldn't have had a clue what to do before, during or after, without the knowledge of others, other than letting the body do its thing and hoping I survived.
We generally wouldn't dream of letting a woman go through childbirth and child rearing alone for that reason. We have some instinctual knowledge, like to push when contractions start, but what then? Do we all instinctively know how to care for a child for the next 18 years?
As a kid I only knew about sex because I'd seen porn and I'd gone to sex ed in school. I know how my body works (from experimentation), but would I know - instinctively with no other input - exactly what I was supposed to do in a sex act? No. Two people shut in a room together would eventually figure it out - but that's not instinct, then. That's experimentation.
I had this friend from a Muslim country when I was in school who talked about finding an old Arabic instructional sex book in her parents' bedroom. I remember I found it bizarre her parents would need a book on how to have sex. Then I remembered in their culture, there is no sex ed in school, parents don't talk about it with their children, porn is banned. It's taboo to bring up. So in her parents' home country, there were how-to sex guides given to married couples because many don't know how it's done. They've never seen it, never talked about it.
I was talking specifically about humans when I said the mechanics of sex are not instinctive. Sexual desire is instinctive in us, libido is, but actually knowing all about how to do it isn't all set out in our minds. We take it for granted we know all about it because our culture contains all that stuff and we see it as we grow up.
But like I said, I think knowing the mechanics of sex are a different thing to whether or not those mechanics feel right for a person individually. Personally, being penetrated was never very pleasurable for me, and I could never get into the mood thinking about that happening to me. That seems to be something inborn, as believe me, I've tried to psych myself out of it for the sake of various relationships.