Why do women give sex to strangers but then reject thier husband's/ boyfriends?

I was just wondering because so many marriages fail from women cheating or men as well, but most times, women reject their partner and then find it elsewhere. Like my relationship in the beginning, we were open, but since closing it now, she says she's bored and rejects me all the time. But before she would suck me off every day, no questions asked. But now she rejects me. Her excuse is that she was younger then, and now she's older and tired. Then she says until she gets a house she won't put out. So she sleeps with all these men but then rejects me and says I have to do extra. I know other stories like this to though and even when I was younger ans single I fucked women in relationships unknowingly. So just wondering why this happens?
Why do women give sex to strangers but then reject thier husband's/ boyfriends?
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