Since there are far more instances of a partner coming out as gay rather than transgender, i’ ll respond to your assertion from that angle.
Do you really believe that the people who chose partners who years or decades later came out as gay, meaning their partners aren’t sexually attracted to them and desire a same sex partner, shouldn’t feel betrayed and believe that their relationship is a sham? By your thinking, since their partner didn’t change anything but a label, they should just accept the reality that intimacy is no longer a part of their marriage or relationship. Bullshit. It works for you because you’re bi, but for 99% of us, it’s an irreparable breach.