A-chan missed her lips, though. But seriously, how those two aren't "officially" together yet ? even the others outright tell them that they're going out in all but name Xx
It's Ai. She seems to have some kind of aversion to making it real and in Ch. 3 she didn't think of what she was doing as being "gay" but later I think she realizes and tries to stop herself.
As far as the "almost kiss" about ten chapters ago, I think Ai's hormones and the mood began to get the better of her. Whenever she's not getting horny she is trying to not be too gay with Chie. (A little gay because she likes her, but not TOO gay.)
It's hard to know exactly what Ai is thinking or what is making her more gay at times and then trying to resist at others. Maybe because Kishi has to draw this out? So he gives us tidbits and then Ai goes right back into denial?