I did suspect the cause might be coming from a less gendered language.
Apologies for the impending multiple paragraphs, but this particular area of linguistics is of interest to me.
For future reference:
- "Guys" (plural) is sometimes used as a generic term for a group.
However, it's less common if the group is exclusively girls.
- Conversely, the singular "guy" is basically never used to refer to a specific girl or woman; this applies even more strongly for most other masculine terms.
The use of "guy" and "bro" that you refer to is more specific to young North Americans, and much less common outside of those regions and generations.
Similar applies to "dude".
(Conversely, while "fella" is more associated with older generations and less specifically North American, it has basically
no association as a generic term; "fella" is always masculine in usage.)
Linguistically, the broader phenomenon is 'masculine generics', which are definitionally
not gender-neutral language.
The distinction being that 'masculine generics' use a
masculine term as a
generic term.
Whereas gender-neutral language either avoids gendered terms entirely, or uses both masculine and feminine terms together.
For example:
"From each according to their ability, to each according to their needs."
or - less common these days -
"From each according to his/her ability, to each according to his/her needs.",
rather than
"From each according to his ability, to each according to his needs.".
Further commentary and background that's not very relevant to scanlation:
In fact, the use of masculine terms as generic terms (ie: using a masculine term to refer to a group, or to an unknown/ambiguous/hypothetical individual) has also been on a steady downward trend since the 1970s.
(Use of what's termed 'generic he' was mandated in many professional style guides for quite some time, but nowadays those same style guides recommend fully gender-neutral language instead.)
The reasons for the decline over time include:
- It's blatantly sexist to assume that the default person is a man, and women around the world continue to complain about this.
- It leads to less accurate and less precise language, which is generally unhelpful.
- It directly leads to prejudice and bias in contexts like employment; advertisements that use masculine generics receive fewer women as applicants, and interviewers and hiring managers given instructions with masculine generics consistently rate women below men with the exact same qualifications.
Even the use of grammatical structures like "his/her" are increasingly deprecated and recommended-against in style guides, on account of increasing awareness & visibility of non-binary people.
That is to say, that the modern use of "bro" emerging as dialect in certain demographics (ie: teenagers in the USA) is a modern example of the exact same sexism that's contributed to the decline of masculine generics in general.
(Culturally and politically, it's part of a general reactionary backlash occurring in the USA in recent years; remains to be seen whether that linguistic shift will stick, or whether it'll be something people look back on in the same way as calling everything "gay" in the 90s.)
TL;DR: You should generally refer to women with terms that refer to women.
(It's the easiest and simplest approach, because there's basically no way to fuck it up.)