A person asking a question about trans issues may come across as awkward or tactless. If they seem to genuinely want to be educated, that is a very positive step. They don't know, so they ask. It's an awkward subject for some. Hell, admitting ignorance of any topic is awkward for some.
There's a distinct lack of education in trans issues in society. I don't fault ignorance. Blame the game.Willful ignorance is another matter entirely.
I don't foresee any massive trans education campaigns in the near future, so conquering ignorance of trans issues one person at a time is a step in the right direction.
Whether a person's question is tactless or not, "Sorry, I didn't know" IS an acceptable excuse... the first time. We're already marginalized enough, victimized enough, and otherwise misunderstood enough without driving away those who genuinely want to dispel their ignorance.
Understanding and knowledge go a long way toward banishing ignorance, fear, discrimination (intentional and inadvertent) and even hatred. It's not a perfect cure and there are counter-viruses such as fundamentalist Christianity and some RadFem hubs. Why do their work for them?
Sure, inject humor if you wish to. Don't berate the poor seekers of knowledge for their lack of knowledge. A question may be tactless, but they may be unaware that it IS tactless. Let them know that it is, and more importantly WHY it is tactless.
Impart understanding and banish ignorance and the world will be a tiny bit safer each time. The person asking the question may even go on to educate others, especially if it was a positive experience. Knowledge is viral, let's increase the vectors of infection.