Idea to bring on question about sex is great.
And comparing it as a fact, after, is great too.
There is also one detail from the beginning, when Yuni chan slapped Fuuko, and Fuuko says:
"Compared to the kiss we had, or to the kiss THEY had, I definitely felt more honesty and passion from Yuni-chan's slap"
https://dynasty-scans.com/chapters/my_girlfriends_not_here_today_ch06#30
Fuuko saw their kiss and took the pic. She noticed a lack of passion between Nanase and Yuni. It gave her the additional reason to act and start to conquer Yuni.
There is also a situation when Yuni had sex with Fuuko, and she is rubbing and touching her and she compares how she felt Nanase and Fuuko separately.
Nanase was toned, and Fuuko soft (to touch, but there is a possibility that it was about their characters too, one rigid and closed, one soft and open)
Yeah, Nanase is very stiff and strict, not so generous to touch Yuni.
And Fuuko is very open to touch her.
And...we should not forget that Fuuko did not answer Yuni why she loves her, they were interrupted by forceful entrance of Nanase.
She literally entered, grabbed Yuni like her possession (and Yuni liked it!) and took her away from Fuuko.
There are situations left open, there was no real closure between Yuni and Fuuko.
And there is real question how the sex between Yuni and Nanase will happen and what it will look like?
Is Nanase capable to give Yuni the passion she needs?
Can it change "overnight" from complete disinterest by Nanase into mindblowing sexual passion between them?
It would be great from author to show us more "skin to skin" events between Yuni and Nanase, that we can complete the pic.
There were only vague flashbacks of sexual exchanges between Nanase and Yuni and not so passionate kiss exchanges. Really not much physical contacts between them.
Did Nanase really had a GF all that time or just called Yuni her girlfriend and was satisfied with it?