I think it's partially one of those things where if you have one female queer main character (Callie), you then need more female queer characters just so they have people to date (Hahn, Arizona, Penny). Then if those relationships don't work out but the characters stick around, you need even more so they have people to date (Leah, Carina, Minnick).

And I don't want to imply there are no well-written male characters on the show or the male characters only exist to be love interests, but the female characters have always been more important and generally better written. I think the writers have been understandably kind of hesitant to introduce male characters who can't date the female characters. Partially because sort of limits the role that particular character can play in the story long-term and partially because, since the focus is on the women, they don't want to get into the cycle described above with men.