It's really not too cool to say "opposite" gender, because there are many more genders than just male or female. Instead, try using "another gender"! It's much more polite to people who don't identify as male or female (myself).
I thought Ruby was a boy. :/
She just doesn't look very feminine to me, so for the longest time I thought she was a boy lol, until I looked her up........mind = blown.
Thank you! If you ever have any questions regarding the issue, please feel free to ask me! I'm always willing to help someone better understand gender expression!