I assume this hasn't occurred yet because international travel has only recently become common.  Even if we do't all end up identical is it possible we will all be the same skinned, the same haired and the same built?

I realise that women need to be physically strong enough to bear children, so puberty is necessary to adapt their bodies to childbirth. But if the purpose of life is to reproduce, we would be able to do it more if men could reproduce from birth

At birth, take a male and a female human from each of their parents.

Raise them together, but independent of any possibility of social learning - other than what they get from interacting with each other.

I.E they aren't taught a language, there are no other humans around, and thus no pre-existing social conventions to mimic.

And here's the question: Do they know what sex is and how to do it?

It's almost inevitable that they will independently discover:

1. touching their reproductive organs feels good,  
2. they are "attracted" to the opposite sex 

But there's a rather large leap from that, to:

1. the male innately knowing that he ought to penetrate the female until climax, and 
2. the female understanding that when that happens, she will get pregnant and produce offspring. 

Are there any studies that have tested this on mice or some other species?

During a lecture on vertebrate reproduction, we were told by our lecturer, that no animal can stop birth. Is this correct? I have been told once before that in the wild Zebras can stop themselves having contractions during the day so they are not suseptable to predation and will only give birth during the night. Is this true? Are there any other mammals that can do this?