Why do women just drop a loyal, and good boyfriend or Husband that treats them right?

Most guys I know usually put loyalty and respect first and foremost in the relationship, (unless they find a reason to just not like the relationship anymore) sometimes it just doesn't seem like women seem to care much about that, especially when some tend to have romantic feelings for a backup man and remain in contact with them while being in a relationship that hasn't been bad. Sure the woman and the backup man may not have sex but still she's having romantic feelings for another man while in a relationship that's practically cheating and as she remains talking more and more with the back up man she puts less and less into the relation with her man that she actually is in a relationship with. Then she just finds some reason to end the relationship with the man that hasn't done anything wrong. Because of this I don't find it surprising that most men don't want to marry because of this.
Why do women just drop a loyal, and good boyfriend or Husband that treats them right?
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