You're right, as a non-religious person, I do believe the Quran was written by humans.
For me, this counts as sexism:
Qura 4:34 Men are in charge of women by [right of] what Allah has given one over the other and what they spend [for maintenance] from their wealth. So righteous women are devoutly obedient, guarding in [the husband's] absence what Allah would have them guard. But those [wives] from whom you fear arrogance - [first] advise them; [then if they persist], forsake them in bed; and [finally], strike them. But if they obey you [once more], seek no means against them. Indeed, Allah is ever Exalted and Grand.
The idea that men should be "in charge" of women is wrong to me. Neither sex should be in charge of the other, in my opinion. Also, notice how this verse says "by right of what Allah has given," not "by right of what I have given." It refers to Allah in the 3rd person, meaning that Allah wasn't the one putting those words on the page. It was written down by a person, almost certainly a man. This is an example of what I mean when I talk about men legislating for women.