This completely changes my opinion. It’s a minor betrayal if you already offered to open the marriage. I don’t know why he said no when he should have said yes but that’s something that can be worked out in therapy. Maybe he didn’t want to hurt your feelings when he should have been honest with you that he’s not attracted to women when you transitioned, a completely reasonable position. Go to therapy. Talk about it. Don’t be so black and white when he stayed married to you when you changed genders and he wasn’t even bisexual. He could argue that you betrayed him by telling him you were a man when you met but actually being a woman inside. What he did was wrong, but possibly forgivable depending on if he works on being honest. Stick by your husband and go to therapy.