well, there is a heated argument about birdo being a boy. i say she's a girl and isn't gay, and nor is yoshi. i support her in every way. the answer is yes. they love each other and do everything together.
Edit: In the Super Mario Bros. 2 instruction manual it said that Birdo is a cross-dresser. Also there is no evidence that Yoshi and Birdo are dating.
Birdo is Yoshis girlfriend, and they are in multiple games together.
No. Yoshis and Birdos are two completely different species.
Birdo
Birdo
Yoshi is 10 years old i know hi is because it was his 10th birthday and Yoshis friend, Birdo is 11
I Think That His Brother Is Birdo But That Might Be His Lover
No. I know it's weird that he can lay eggs, but all the other characteristics about him show that he is a boy (like being married to Birdo although Japanese Nintendo made Birdo a gender confused male).
There is no yoshis valley
Birdo loves Yoshi
There is no black birdo.
The official answer would be NO, there is no proof in-game or in any Nintendo statements that Yoshi has a girlfriend or any such relationship. There however is some speculation that Birdo, with her feminine qualities and apparent similarity to Yoshi's species, is his girlfriend. There is unfortunately, no evidence to back this up.
Birdo and Yoshi are neither family-related nor lovers.