Im quite confused here but i think i got it:
Its all in their head and its just regular lesbian sex.
Though the artist ilustrated it well.

Base +8, wanna +13, Hyoros +10, TheOldDeus +7

Not really futanari, as its just their fingers that they are imagining as dicks

Base +6, thenewdrax -6, Bomb Square +8, 57913 +6, TheOldDeus +7

I suppose lesbians don't imagine their girlfriends fingers as dicks like that to crave them, but who knows, I'm not one.
I say it's quite of a bad taste in this doujin where instead of girls fingering you see some big penis with balls and whole dialogue is like author just got it from some typical hetero doujin, kind of ruined whole scene for me.
But art is nice in this in overall despite needless annoying penis addons which didn't make any sense.
Still it's better than some typical faceless hetero rape for doujin with a real yuri character included.

Base +8, MiscMan +1, mitsuitony -7, wanna +13, funnydingo +1, gengaozo -6, InsaniacPsyche -10, HXW -9, Romi the Byzantine +7, Bahamute +17, Semro_ +4, Soyungundam -6, 57913 +6, and 3 more...

didn't understand the futa tag...

Base +9, TheOldDeus +7