Why did I have to open my mouth?

This week I had a new student in my class and when they introduced themselves they had hair around their face and to their chin, a high voice, and their English name was joyment. So I thought they were a girl. I handed them my attendance sheet and asked them to write their name in pinyin, their english name and their gender. That was plenty of information for them to fill out the form, but I had to keep talking and when I said write your gender I said and obviously female. There was no point for that comment, and when I got the paper back I was in shock b/c it said male. I thought since this is a lower class that they were just confused and did not understand the word gender. As the class went on I realized that he really was a male, and I felt terrible b/c he is so nice and answers questions in class and was very friendly and I called him a girl. Why was it necessary for me to say obviously female!