1) "We are modern people walking around with brains built for the plains and the forests of distant ancestors."
We are made to live in plains and forests, as our ancestors did, and our brain is made to work best in that environment.
2) and what about "we have tendencies to action that need no longer be our master"?
Some instincts/desires are hardwired into us, though we don't need to yield to them anymore (eg. to survive)
3) What is another way to say "men and women are roped to the poles of their sex"?
Men and women are inexorably bound to the distinct, built-in characteristics of their sex