Umm, unless I am missing some vital info, I didn't think they ever agreed to like go out as girlfriends. They are still both in the pre-relationship part where one girl in particular is questioning her own feelings towards the other girl, you know? Probably been in only straight relationships in the past, but suddenly cute girl she hangs out with becomes even cuter and maybe now shes gay or something type of feeling that Mizuki-san is going through. I relate too much to the feeling.
EDIT: And this is why the grabbing her pinky is such a big deal!!! One of them as actually reached out to kinda "test the waters" if you will. Yukawa-san is looking to see how Mizuki-san will respond, and honestly, this is the part in Yuri is live for... when two people realize they like like each other.