Same could be said about you - I always find it very sus when someone gets trigger so hard by a character that kept their virginity/ isn't having casual sex.
It might not be common these days but it's an ideal to aspire to; as well as a cultural difference. Manga characters usually aren't average (and thus boring) for obvious reasons , but wouldn't complain about them being stronger/ smarter/ superhuman even or more attractive (well you might when it's a female and you are certain ist). Thus you are the sus one.
are you even aware of the bullshit you are writing?? what if ppl were together for 10 + years and discovered things just weren't working out and they go their separate ways afterward and then meet the one they were destined to be with - do you really think any sexual relations up to this point were "casual??"
this is exactly why i'm saying that your point of view is creepy as fuck. for you it's either one guy/girl for life (something they apparently have to figure out in school/college already where they still may be minors) or they're manwhores/sluts sleeping around with everyone. this close-minded bullshit doesn't allow for anything in-between.
and what the flying fuck do you mean with "ideal??"
an ideal is something both sides strive for and it's where they are at their happiest. if that's a chaste life, sure. if they genuinely want to go the next step(s) - either before or after marriage - that's their own fucking business. all of this is assuming you live in an area that even remotely resembles a democracy (not just on paper) of course - and both genders have equal say in the matter and no "cultural differences" (lmao) are interfering with that.
lastly, of fucking course are manga characters boring. each manga rehashes the same fucking tropes over and over.