There was a LOT to love about this episode, but one of the things on the list was early on when Kara suggested Mon-El join her on her journey and he checked to make sure that she was really comfortable with that and you could just see in her eyes that she reached a point of love and trust for him that was just an absolute for her. You could see the endgame in her eyes.
I love how antis conveniently forget that Imra was the one who told Mon-El to go figure out his feelings and gave him the green light to stay with Kara if he realized that it was his destiny. So no, Mon-El is NOT a cheater, he’s NOT doing anything behind Imra’s back, and for all she knows her marriage was over the moment she sent Mon-El back to Earth. So that’s that.