Speaking from personal experience and having been on both sides of the fence, the last thing his parents will be thinking about is what was or is twixt your legs. Once he announces to them that he is bringing his girlfriend to meet them, it sends a signal that there is a possibility they are meeting their future daughter in law. They will be curious about you and your background such as your parents, where you are from, do you have good manners, sensible, education, job, etc., and if they don't come out and ask any of these things, you can rest assured they are thinking them. Our son recently brought home his girlfriend so we could meet her and vice versa. The last thing on my mind was what is her vaginal or lack of vagina history -- did not even think it... and she really impressed me!!! Love her and am so happy for my son. Naw, they won't be thinking what you are thinking -- trust me.
Now that you've asked this question, I am wondering what I would say or do if my son brought home a transgender girl to meet us. Would we tell him about me? would we tell her about me? oh my -- who knows and I'll cross that bridge should I ever come to it.