Whenever ONS would happen to any of my friends (girl or guy) outside of China I'd just hear a story like "we had a last tequila shot around 3am then went to his place. Breakfast this morning at his house. He's pretty cool" end of the story (well unless something more fun happened!)
Here in China many girls I have met (and will meet also for sure) can ask me about how many times I had ONS (more than 5, less than 10, so you don't need to ask me later), and the first thing you find on half of the girls profiles here is "Don't add me if you are looking for one night stands".
Last time I had a french friend feeling bad about that, I think she (of course I've never heard a guy complain about it!) was 16 or maybe 17.
So any insight on why people feel (or pretend to feel) so bad about it might be interesting.
I have a beginning of answer myself that I'll share with you later, I want to have other fresh opinions before.