The natural law doesn't say that men and woman have equal roles either. Whatever they are, men and women are not equal in every sense. By nature, for example, they aren't. This is obvious.

Now, both sexes are equal in human dignity and rights. But human interaction and nature institutions ultimately create hierarchy. So, some of the more prudent reasons ancient people thought that wives should be subject to her husband is

1) woman tend to be more sentimental and thus irrational

2) when there is a disagreement in a marriage, a conflict of deep importance that requires a decision, in order for the marriage to continue, one spouse needs to "veto" the other. The man should be the ultimate decider because of (1) and (3)

3) Woman are biologically inferior to males, because they are a "misbegotten male."

(1) is arguable, at least. (3) has been proven wrong with the advancement of biology. (2) points out something very important, and probably explains why divorce is such a huge problem in our society.