Well if you believe that, then you admit that there is a difference between men and women (within your kind of logic).

Of course nurturing has an effect, but the fact remains that men and women are different and have to face different challenges in life. It is not only the 9 months of pregnancy - yes, men could stay at home after pregnancy (now that we have breast pumps for the milk). But there are still several other differences, as I explained elsewhere, not the least the problem of finding a mate to begin with. It is a completely different challenge for men and for women, simply because women can only have a few children, whereas men can have thousands of children in theory. So women have to be more selective etc. Again just one example, but in the sum things like that end up evoking behavioral differences.

If all behavioral differences were only the result of nurturing, then why are there no more random variations? Why are there not more societies where all boys behave like girls in our western world, and vice versa? If boys and girls really were equal except for the nurturing and their visible difference, then we should see such variations, because if a society with our boy/girl pattern is stable then the reverse would be equally stable.

Edit: I think you yourself mentioned tomboys, so there are people with reversed roles. Therefore, all else being equal, whole islands of societies should develop where roles are mirrored (because apparently the reverse strategy is being evaluated on a regular basis - so if it would be equally successful, it should from time to time become dominant). But they apparently don't develop.