What does Abigail mean by phrase “vassals of your sex"?

Abigail is referring to the belief that men are superior... that women are subservient to men.... subordinates. Note, Mrs. Adams was NOT subordinate, nor did her husband expect her to be.

Men of Sense in all Ages abhor those customs which treat us only as the vassals of your Sex. Regard us then as Beings placed by providence under your protection and in imitation of the Supreme Being make use of that power only for our happiness….”