It wouldn't be an Ikeda Takashi yuri if there wasn't at least a little bit of angst now would it? Interesting to see where this casual relationship goes, if this will change anything in the long run.
Ataru's line saying she can afford to take things slowly because Kyouko's hot, especially coming immediately after Kyuoko's line, makes it pretty clear that she's angling for a more serious relationship, despite her tough girl act. Similarly, Kyouko's clearly dissatisfied with the state of her faux-relationship and one sided feelings.
The current state of affairs in unstable, so I think it's almost guaranteed things will be changing sooner, rather than later. That change could be Rei deciding she has actual feelings for Kyouko and the two of them getting together for real, but that seems unlikely without some sort of major shakeup and because it would be a real anti-climax for Ataru's ark. I think the most likely scenario is Kyouko and Ataru get together and that causes Rei to realize what she what she could have had and how much she regrets the missed opportunity.
That said, I'm really rooting for the dark horse option: Rei realizes her feelings, but and all three of them go the poly route.
