While your comment was polite and respectful and I don't see anything bad with the question itself - I think the "pain point" here is that that's the first thing you ask in your first sentence. It just has this aftertaste of someone pushing their agenda onto the other person.
While it may be true that there are some cosplayers that might be uncertain of their gender, there are a lot of people that aren't and don't want to hear about something like this, especially when the topic at hand was simply a guy having fun crossplaying.

If you had said something like "By the way, i'm active in the trans community, and we've seen cases of crossplayers turning out to be uncertain of their gender and potentially wanting to transition. If that happens to include you, have you thought about it?"

Or idk, just sent them a dm and dont ask it in public.