Hmmm... could've been a case of "you notice that which is on your mind" though, maybe? It's like that famous phenomenon when you're afraid you might be pregnant (or hope you might be pregnant), and suddenly you see women with babies everywhere. In this book, Harry became consciously aware of his attraction for Ginny and wanting to kiss her, so he noticed all the others kissing. Hm?

Of course, you could say he didn't notice it like that back when he was attracted to Cho, but I don't think his attraction for Cho was as serious as his attraction to Ginny.