Him cheating doesn't mean he doesn't love you, value you, or want to be with you. But if what he truly wants is open relationships, the two of you are going to be miserable staying married, even if he never acts on his feelings again.
Come off it @dustofneptune
I have been in polyamorous relationships myself and agree about them being emotionally exhausting, and most of what you say until you get to this point.
Him cheating absolutely means he doesn't value OP.
Whatever you think about polyamory, it's not the same as cheating. If you cheat, you lie to someone, do something behind their back, you are screaming out that you do not value them, you won't give them the basic respect and courtesty to actually speak to them.
If he is 'polyamorous', he should have spoken to OP before having an affair about how he was feeling, so that she was then empowered to make a decision about whether she wanted to stay with him whilst he sees other women.
What he actually did was extremely controlling, manipulative and puts OP in a position where she has no choice. He betrayed her and that does not 'value' her. He might want to be with her, he might even love her, but he has very little respect for her.