From what I can see, at least part of the concept is that Reimu wants Yukari to feel like she does, since just fucking without that feels empty her. Yukari claims that, as a youkai, she has no such feelings. However, when Ran comes by she reveals that Yukari doesn't actually think like other youkai but much more like a human, showing that what she said the previous night was a lie. To underline this, she has Ran pass on a message that Yukari is going to go start an incident involving Marisa (or sexually harass Marisa, it's not too clear there) in order to make Reimu pursue her.
So the whole thing is flirting in a way. At least that's what I take away from the story.
I do like that Ran acts more like a fox spirit in this than usual, with the shapeshifting and imitating birds and somewhat inhuman voice (indicated by her word balloons).