It's the male sex who are cruel and deceitful - they cannot keep oaths, and, the chorus prophecy, it is women whose reputation will turn over time and eventually become well thought of. Remember, too when reading the below, that the chorus is all-female itself:
Men have tricky counsels, and their faith does not stand firm in oaths, whereas my reputation will turn and have good fame.
Honor is coming to the race of women. No more will ill-sounding fame hold women.
Why have poets always spoken against the faithfulness of women? Euripides doesn't actually precisely tell us: what he tells us is that, when honour has come to the female sex,
The Muses will stop hymning my faithlessness in their old songs, for Phoebus, leader of songs, did not give to my comprehension the inspired song of the lyre; since I would have sung a hymn against the male race.
The reason, in other words, that poets have always made anti-feminist poems is because Phoebus (god of poetry and music) would not inspire any poets to write hymns against men. Phoebus, in short, only inspired men to write poems against women. So we don't know why poets have said otherwise: what we are told is that the counter-argument had never previously been voiced.