@Elcief_SSN Not really a Japanese thing but more of a teenager mentality. The idea of being "inexperienced" is usually something someone young considers embarrassing, even though they're probably the only one thinking it.
@Kalocin I think those teens should more embarrassed for having the idiotic idea that not having a gf/bf means being inexperienced and embarrassing. Don't know how many teens think like that (I never did and I just turned 18) but Japanese mangaka sure use this joke a lot. Also another one where being a virgin = something embarrassing (Highschoolers included).
@Elcief_SSN I think it is just us not wanting to be left out. And because of others bragging about this and that, and having your friends get shocked by you being a virgin and whatnot (even they don't have any intentions behind their reaction) making you feel like it is bad to be a virgin.