She didn't know that at the time.
OP, it's because historically, men have been allowed to cheat while women were at home saddled with like 10 kids. Rather than leave the husband, the woman usually stayed with him as long as he was supporting her and the kids. She had a low earning potential and several mouths to feed. Women pass this advice to stay with cheating men to each generation.
I remember one of my ex-boyfriend's grandmothers telling me about how her husband cheated on her. She said that her mother told her to stay with the husband and to stop physically fighting him as long as the husband was taking care of her and the kids. I didn't think about it at the time, but she was conveying a message to me to let her grandson play around all he wanted.