Actually, unless I'm severely mistaken, most people managed the get laid over the last tens of thousands of years, but they're mostly terrible at understanding human and relationships. I've seen too many relatives, elders and generally people around me making one too many mistakes -- some even seems to do everything wrong. And I'm rather scared of that.
"Getting laid" is definitely a skill by itself, but I disagree whether it has any relations with understanding human beings at large.
I generally wouldn't suggest learning about sexuality first-hand from ones relatives. :|
So, once upon a time I too was very much concerned about making one too many mistakes--part of learning is figuring out that those mistakes are a)somewhat expected of you as a maturing sexual entity and b)harmless.
As long as you treat other people with respect and try to be a good person, your romantic screwups (and you will have romantic screwups) will just be slightly stressful learning experiences.
One thing that I've noticed is that it seems modern intellectual and enlightened internet culture is the worst possible place to discover and explore your own sexuality.
Abnormal urges are treated as standard, and normal urges are treated as discriminatory, barbaric, unseemly, or criminal. Go interact with normal people at bars or coffee shops or social events and see what you can see--it is going to be very hard to find positive role models online in the places you'd normally expect wise elders.
Hell, you're honestly better off looking through 4chan than you are any of the conservative, progressive, or technical sites here.
You can avoid most major relationship mistakes if you actually care about your partner, as in, as an end-in-themselves rather than as a way to get laid. Unfortunately, this is the part most people miss.
I guess a lot of people don't realize how dumb their selfishness really is.
angersock mentioned sexuality specifically rather than non-sexual relationships. Getting laid doesn't help as much with non-sexual relationships and it doesn't help as much with non-sexual aspect of human beings. For those, I still agree with the advice they should be out interacting with other people instead of reading books, which means interacting with relatives, elders and generally people around you.
Bingo--author mentioned sexuality, which is something that is specifically best left to fieldwork instead of reading Wikipedia and books...and yes, I'm speaking from experience.
Talking to people at parties and raves and orgies and meetups is going to give you much better experience and understanding.
And as for the non-sexual relationship stuff, I'll attempt no general claims at this time.
If somebody's goal is to understand sexuality, they should be out interacting with other people instead of reading books.