This is one reason I love Higashiyama Shou so much. He's so good at the subtle character interactions. Ichika clearly still young and part of her is worried about Aria getting drunk and yet she's still tempted. Then, as youth often is, she's quick to take advantage and say "sorry Khara, we're going alone, no room for you!" And her wanting to kiss more but then discovering that even though alcohol can show her some amazing things...the burp.
Normally, even when it's clearly a mutual feeling like in the case of these two, adding alcohol to the mix still makes me a bit uncomfortable. But in this case it's clear that Ichika's youth is playing a role and it's something she can learn from as they grow closer.
Of course while Aria is clearly affected I'm not sure how much it's her losing control and how much it's her using it as an excuse to do things she normally wouldn't. Part of my suspects she's not as drunk as she's showing. Either way, I'll bet that their night ends here, leaving Ichika with mixed feelings.
Anyway, OH, MY HEART