I don't think anyone is arguing that "guys" would be accepted usage to refer to a group that's entirely female gendered, only that it's acceptable for addressing mixed company. A bisexual person may well think that way about their sexual history in toto -- or they may find distinguishing genders is useful in this context.
This really depends where you are in the world.
It's true that my female friends don't use "guys" to refer to friend groups that are entirely female. The word they do use is "lads". Which, to UK people would sound even weirder than claiming "guys" is gender neutral, yet it often is here.
It's a peculiar thing. I've seen guys get offended at the assertion that "guys" is gendered, and then enraged at the suggestion that they may have slept with "guys."
"I only date guys" - unambiguously saying you go for the male gender
"Hey guys" - a gender neutral greeting
Dick can be a man's name, a detective, an anatomical reference, or an insult. Somehow we manage not to feign outrage at a Richard's nickname
Ah, see, nuance! So often when this topic comes up, there is none of that to be found! Hence, people asking obnoxious questions like "how many guys have you slept with." You see, there is a contradiction: people get offended when their words offend others, and they exhibit this offense with kneejerk responses. But what's happening there? The original speaker is getting offended by the others' words.
I would also bet that the straight male would say "no" to the question "do you fuck guys?"
https://www.merriam-webster.com/dictionary/guy
It's exactly like the evolution of the term 'bitch'. Its origins are gendered but its predominant application now has nothing to do with canine sex or mean women.
"how many guys have you had sex with?" implies maleness, though it would also depend quite a lot on context.
"what's up, guys?" is used in a gender-neutral way, therefore it is gender-neutral. That's an entirely descriptive, not prescriptive, observation.
Language, especially slang-y words, are more learned than taught, and neutral-guy is no exception. We humans use "guy(s)" as male in some context and neutral/unspecified in other contexts. No one told us to do so.
Whether we should consciously stop doing so is a judgement call. When someone actively makes that correction in conversation, that rubs some guys (including my bio-female partner) the wrong way.
Now, I think it's noble to attempt to use gender-biased language (such as neutral-guy), and eventually it will hopefully fade from usage, but it's still obnoxious to make the correction by interrupting conversation.
~ Humpty Dumpty