I can believe she was drunk, otherwise I don't think she'd be so stupid to kiss anyone in public.
But public drunkenness/smoking in public is usually as big a faux-pas for idols.
Being drunk the first time might have been believable, IF she had only gone out with the guy one time. However, how do you explain the fact that she admitted to going on multiple dates with the guy? The Shunkan Bunshun article mentions they reportedly went out on at least 3 dates within a 3 week period, and who knows if the two of them went out on dates where they just didn't get noticed/caught? She couldn't have been drunk during every single date. Safe to say that she was at least careless, but under the circumstances, you cannot objectively look at this situation and honestly say that, especially after going out with the guy for (at least) a second and third date AND getting caught kissing him, that she didn't like him/have feelings for him nor can you say that she did not know what she was doing. She took a chance, and she's gotten caught. How she and her management/producers deal with it from here on out will be the thing to see.