Why do women say their new partner is toxic after 1-3years of dating?

I don't mean to come off as sexist, I'm just really confused. I have women in my life who would say their boyfriend was perfect, but month's later say he turned toxic. How does this happen? One friend boyfriend would bring her lunch every day to work but then turned super controlling months later, and all the romantic stuff stopped. Another girl boyfriend was super romantic, but around the 15-month mark, he stopped taking her out on dates and the relationship become all about sex no matter how much she told him this made her feel it didn't change. Another girlfriend cheated on her at 17 months.

I would learn to her everyone's experiences about this
Why do women say their new partner is toxic after 1-3years of dating?
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