You need to forget it. Yumi and Sachiko are dear to each other, close as sisters, but not lovers. It’s a story about this relationship – close as sisters – that is the point of almost every relationship in the series. That is why they are “soeur”.
WHAT!! marimite is not yuri!!!??
Why are people suprised about that? It's class S yuri. That's literally Yuri Bait - The Genre.
I know this conversation is somewhat old.
But it's the contrary: yuri, in general, isn't about lesbian, and by this, I mean “lesbian” in the LGBT political and social ways like we have in the West.
But the yuri absorbed the lesbian stuffs, not the other way around. And thus Marimite is far more a “traditional yuri” than something like Morinaga's Girl Friends, which is pretty much lesbian.
If you heard the Japenese people behind the yuri, they say something like “yuri is about spiritual connection” or “something between friendship and love”.
In a more western way to say this, yuri is about homosocial bonds between “women” (well, mainly “shōjo”). The homosexuality we can find in the yuri is simply an extension of this homosociality.
That's why in yuri, we have mainly girls (shōjo) and not women, rarely marriage, rarely "I'm a lesbian", next to no LGBT militantism or homophobia. Because yuri isn't about lesbian in the majority of case.
Yuri is a mixture between the homosexual behavior during the pre-Meiji period, female romanticism in the prewar Japan (the “class S”), and more recently the Western-LGBT-lesbian ideology.
So yeah, “lesbian yuri” is a subgenre of yuri, like “class S yuri” is one another, or the recent “soft-yuri”. And personally, I'm glad that yuri isn't about lesbian-only, but is in fact very diverse. And “class S” isn't a bullshit, it was an odd thing born from three different things: the romanticism imported from the West, the patriarchal society who wanted to “preserve” the shōjo in order to have “good wives”, and an act of resistance of those shōjo against the patriarchy, so it's partly feminist and partly patriarchal, like the rest of the “shōjo culture” (Takarazuka, Yaoi…). It's a complex and interesting matter, not “bullshit”.
For those interested by the subject: here a quick read about how yuri is more complex and subtle than just “lesbian”.