Theseus has conquered Hippolyta's Amazon nation and has, in essence, "won" her. Hippolyta, it is argued by some critics, is not extremely happy about her marriage to Theseus because she remains silent through much of Act I. I can imagine she wouldn't be too happy about having to marry a man who conquered her people:
They believe that this silence does not reflect Hippolyta's happy acceptance of her marriage to Theseus. Rather, her reticence suggests that she has been coerced into the marriage (remember, she has been taken captive), and that she seems to regard it with resignation and sadness.
Hippolyta is was once the queen of the Amazons, which is a tribe that consist only of women. After conquering the Amazonian, Theseus fell in love with her and got engaged with her. Her status as Theseus's former enemy perhaps mirrors the young lovers often contrary relationships.