@Aethelwine I can completely understand the transgender person wanting to simply be accepted as the gender they live as. It makes perfect sense. As I said, once a person is transitioned, to me they just are that gender, no need to refer back to what the person used to live as, unless the person themselves wants to for some reason.
Going back to the Q, I don’t see any reason to know gender to friend someone on Facebook. However, I will say our language unfortunately uses he and she and him and her a lot, so it’s good to know someone’s preference if it’s not easy to tell how they identify.
I also think it’s good for transgendered people to not be easily offended, and I say that to everyone, not just transgendered people. Too often we (I use the big we) are so PC we are disgusted with people when they mean no harm. When people ask me if I’m jewish I don’t assume they are antisemitic, when people ask where my husband is from I don’t assume they are bigots, when I thought that a friend of mine was a lesbian she wasn’t offended, when my gay BIL’s thought one of our friends (we all know him) was gay, he didn’t care, when I assumed a new friend was straight, he corrected me to tell me he didn’t have a wife, but rather a husband. But, I have the luxury of living in circles where most people don’t care.
Being transgender might be in a separate category than most of the other things I named, I’m certainly willing to accept that. The transgendered people I have known didn’t seem very uptight about it, but I didn’t know them very well. The two I did know, who I saw on a daily basis, I worked with. It was in the Cosmetics industry, and they were all glammed up, not just trying to discreetly blend in. They looked more like a stereotypical transvestites.
I wouldn’t just ask a stranger out of the blue anyway. I was referencing my BIL, and people I have worked closely with, etc. even then, it’s not out of the blue, it depends where the conversation is going.
Always better to err on the side of not asking if there is any reason to think it might be a bad idea.
@marinelife’s link says to just respond “that’s personal” I guess people can just do that when an inappropriate question is asked. I understand the feeling. I can’t tell you how often people ask why I never had children. I hate that question.