Yes, you're right about starting with two people. In many fandoms I've gone looking for favorite-male-character/female, and there's nearly nothing. There aren't any female canon characters not already defined as love interests for another male character, or for some other reason out of the running, and the BotWs or OFCs are completely unequal to him in terms of characterization, and unless the author feels like writing a whole novel developing her, necessarily will be.

The same thing goes for F/F fic, only here it's two characters in dire need of character-development. Unless a show has a particularly strong mythology, you might as well write original fic.