I think that the butch/femme is more a real life stereotype. But when I read/watch/consume any kind of fictional narrative or media, the femme/femme is the more prevalent couple of the lesbian world. Maybe authors think that they'll be more accessible by heterosexual people or that they'll at least be both attractive to heterosexual males (who are probably the main consumers, at least in Japan, of lesbian works).
You think so? I've always gotten the impression that yuri never became as big a genre as BL precisely because marketing it to heterosexual males never really caught on the way marketing BL to heterosexual females did. Rather, yuri seems to be, to this day, mostly marketed to females, both gay and hetero--gay ones for obvious reasons, and hetero ones who, I dunno, like certain qualities in their romances that don't seem to work as well when there are guys around. And OK, sure, a few guys with good taste, but guys with good taste aren't a market.
That's why for the most part, the artistic and plot style of yuri seems to come out of shoujo and josei rather than shounen or seinen. There are exceptions, they often stick out like a sore thumb, of clearly shounen-oriented yuri where somebody thought "Hey, maybe the boys would like to see all girls in the story so there will be 100% boobies", but it never seems to have caught on very well--perhaps because with the prevalence of the harem subgenre in shounen, the boys already get like 80% plus boobies and a viewpoint character they can project into. Even though it would technically grow the genre I'm just as pleased those never took off, because stupid shounen fanservice manga are stupid and I don't want them taking over my yuri.
While I'm happy to know that I have good taste (despite not qualifying as a market group) I will mention that most fan service style harems make me wince. I prefer to actually have a quality plot in the stuff I read and the lack of plot+rather unintelligent/unreliable male protagonist make most harem style manga a no-go for me.
Tell ya what, if you can round up some people I'll get some pitchforks and torches and we can go "convince" the publishers to stop releasing so many shounen fan service manga.