I did not say "mankind is fine" I said that it's etymology is not sexist. The word is not "mankind" because only men count, the word is "mankind" because when it was made up "man" included all people.
That said, it's frustrating when perfectly good words like mankind are labelled as offensive because people either misunderstand their origins, ignore the intent with which they are used, or are simply looking for reasons to be offended.
If you went into a restaurant with cutely-named bathrooms, and went into the "guys" bathroom, you would probably be surprised to find women in there. If someone described The Spice Girls as a "group of guys" that would likely be more confusing than someone describing The Beatles as a "group of guys"
It might sound completely normal to your ears to use "guys" in reference to any group of people because you're so used to responding to it (and, yes, I Internet-stalked you a bit to confirm your gender). Having never personally identified with the word "guy," however, I, at least sub-consciously, and sometimes consciously, think it's odd when someone calls me a "guy." I mean, I'll absolutely respond, and won't say anything to the speaker, but I'll feel a tiny bit put-off, and maybe a little self-conscious, because I'm the only person in my company who isn't actually a "guy"
Like the author also brought up, there are plenty of alternatives to "guys"
Seems mankind, from etymology, isn't sexist. It derived from the neutral word `mancynn` in a time when wer, and not man, was used to refer to males.
At the same time, bringing attention to the implicit sexism (a la 'mankind') is valid. It doesn't mean anybody has to shame themselves for saying 'guys', it just suggests that they might want to consider other word choices in delicate situations or when they've got the reserves to be a little more conscientious.
"You guys" is the most common form of "you" plural used in the mid-west and western United States. For those who were not taught English in a region where "you guys" is standard, I can assure you that we consider "you guys" to be completely gender neutral. I can see how this phrase would seem strange to someone from a region of the United States that uses "you" or "you all" or "ya'll" or "folks", but for us it is simply a phrase that is used by both women and men when addressing a group of people of any gender (all women, all men, mixed).
See you guys tomorrow != See you men tomorrow. Are you guys done yet? != Are you boys done yet?
Now, is the etymology of the word sexist, a la "mankind"? That I do not know (it probably is), but the intent of those of us who use it is simply that it is two words meaning "you all". We do not think about injecting gender at all into the conversation, in fact we use this phrase to avoid it (as silly as that may seem)! I would imagine that this phrase is more likely to turn heads when it is used in a region of The United States where it is not the normal linguistic way of saying "you" plural.