He doesn't want to have to see your parents, his in-laws, because then he has to come face to face with the unpleasant consequences of his actions/choices, and it will make him uncomfortable, which he'd rather not have to feel.
With my ex it was our son. When he told me he wanted to transition, and was so excited about it, I asked him, "What about [our son]?" He wouldn't even reply. I asked numerous times, and he simply refused to engage the question. Too much cognitive dissonance. I was stunned and infuriated, just as you are. They really are on beyond inconsiderate!
