Well, I don't read many Chinese comics but I've seen it in lots of novels. I guess there is some truth to this in so far that the FL would likely not have willingly sex with anyone prior to marriage (often not even with the ML) because, apparently, you're a slut if you do. Then again I can also come up with two novels on the spot that would speak against that justification:
In one there's lots of rape in marriage happening (with the guy the FL loves oh sooo much and everything will later turn out to be a huge misunderstanding as far as I've gathered from the comments) and then there's another one where the FL is raped before marriage (by her later husband but they were both drugged at that time so it goes both ways), gets pregnant and her family finds it to be a disgrace so she's sent abroad. It doesn't really matter whether she was raped or agreed willingly, she lost her virginity and that's a no-go.
So, even though I think it's a nice explanation to say it's because her losing her chastity that way will be (more) alright, I really don't think that's the case for most stories. To be honest, I think that this is rather a problem with depictions of relationships in Chinese media in general.
Like, they tend to have really, really uncaring male leads - very often downright abusive stalkers or at least men who don't care about women in general and will treat almost everybody from their wife, their family to their employees like shit - and the women are being forced into marriage by either their families or the ML himself before they slowly fall in love. The FL almost never stands up to the ML (or their family) regardless of what happens and if she does, well, that's culturally unacceptable so she's a bad person.
I do think that this is grounded in culture. Like there's this old belief that a woman is her family's (read: father's or if she doesn't have that than it's the brother, grandfather, uncle or whatever next male relative) and after marriage her husband's possession. So women are, very often, treated more as commodities and basically sold off to a guy.
The worst is that this is still true even today to some degree. Like IMO women are basically treated like 2nd class citizens. Just have a look at the child policy where you could only have one child - unless your first child happens to be a girl, then you might be allowed to have a second child because god forbid you only have a daughter. They're only good for wasting money and marrying off to form connections *sigh*
I'm sorry. This turned into a bit of a rant