When writers of this period use the term man, sometimes they actually are referring to all human beings, men and women.  Other times they really do mean MAN, or men.  In the case of Milton here it is not entirely clear which way he means it.

One possibility: the fall occurs when Adam, the man, takes the bite and thus disobeys God.  In this view it is Adam's disobediance which signals the fall from grace.  If Adam had refused, though Eve had already tasted the forbidden fruit, then there would not have been a fall.  God, instead, would have just created another partner for Adam. It is the man's actions here that are most meaningful.  Another possibility:  Milton actually means "human beings" and is including both men and women in his term "man's."