Do you think there's ever a valid reason to have an affair?

I'm watching this show about two couples who each have an affair, it isn't your usual sappy romantic drama but it actually has a lot of emotion and reason in it and it really made me think, if having an affair can be justifiable. Obviously, I'm not married so I wouldn't know on a level like that.
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What I mean when I say 'reason' is either the married couples are in an abusive relationship or one where they're neglected and lack any love and support from their partner so they go looking elsewhere.
Do you think there's ever a valid reason to have an affair?
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