Lets be real here, it would be a rare case that both people would fall out of love at the same time,so we are talking about in most cases one person ..
If my husband didn't love me,but I loved him,I think I would be rather miserable,I have too much respect for myself and children to stay with someone who didn't want to be around me,And I believe that for someone in that position they would find this fact very difficult to hide,,which in turn is NOT good for the kids.
If my husband didn't want to be with me, I think it would be fairly obvious,for example how would family holidays go?Communions/Confirmations etc etc.I would imagine it would be a huge strain on all involved.
It is not true to say its often an excuse to make the person who walked feel better,because in my experience its often the person who is not loved who decides this is not for them,and who can blame them!