When I played American SMB2 for the first time as a child (after I got over how weird it was), I always thought Birdo was female because it was pink. Then I later played Wario's Woods where Birdo was known as Catherine (which I think is a better name anyway) and wore a bow. I'm so used to the idea of Birdo being female that I wasn't surprised to hear her with a female voice in Mario Tennis. And it kind of gives a reason for the poses of Yoshi and Birdo on the Mario Tennis title screen. I mean come on, that's too coincidental.
I acknowledge that Birdo most likely is a tranny. And after reading a little about Captain Rainbow, I know more than I ever wanted to know about it. But I keep on thinking "girl", and so find it strange whenever I hear a male voice coming out.
Don't really care for Birdo though. Birdo, Daisy, and Bowser Jr. are at the bottom of my list.