All 3 sides are hurt. If the man truly loved one of them, then he is also in a dilemma. He may be acting on his selfishness but he also has his feelings... if they were real.
It hurts more when all 3 sides are aware of each others' existence. Given that all 3's feelings are real...
1st Girl. It hurts knowing you are not the "Only One". Even if you are 'legally bound'. You no longer are the only one in his heart... or his heart fully belongs to someone else and is staying with you merely because of his responsibility and contract or status quo.
2nd Girl. It hurts to be number 2. Number 2 cannot proudly claim the man as his. Especially if she does not want to share and is aware of the moral complications and the judgment of society.