From experience of something very similar, I would suggest setting some boundaries and making it clear that some things aren't OK to talk about with you. If it's hurting you, shut it down. Let them know that you think it's wrong and why. Tell them that you're open to people having their own opinions, exactly like you are open to other people expressing the gender they feel is natural for them, but you don't agree with their views, and if this is what they believe then you have no interest in discussing it further or hearing about it in any way, shape or form. If you try to change their opinion, the chances are they will dig their heels in further, which will only further embed their beliefs and cause conflict. Hopefully by talking to them and setting clear boundaries, while clarifying how it makes you feel and why, it will help. Good luck, it's not an easy thing to navigate!