Sure, it might be common, but dude, the world isn't that way. Look at our government; almost entirely men. Look at our Fortune 500 companies; run almost entirely by men. Look at our richest people list... almost entirely men.
Now, some people might say "Well, that's because women are inferior."
Others look at that and say "Well, yeah. Women generally have not gotten the same chances that men have had over the decades that helped create those different institutions."
So you might say it is "common sense" but in reality our world doesn't operate that way; for centuries the "common sense" has resulted in women not getting the same shots as men in nearly every single avenue of our society.
As for "mankind" being a thing... well, it kind of isn't. It is just an invented term for all of humanity, but yes, one that includes the word "man" and that was created by men in a time when men ran... well, everything. The idea of some women feeling that it is a bit exclusive and wanting to change it to "humankind" makes total sense to me.
I think you and I have lived our life and seen things from our own point of view. There are a number of people who have lived THEIR lives and see things from a different point of view, and I think it is worth considering when they say "Hey, this feels wrong" even if you (and yes, sometimes I) disagree.