Every two-handed mace I find shows a red X for my Monk, as well as a red box around my current weapon in my inventory. The maces do not show class restrictions (example below). Are Monks unable to equip certain non-class restricted weapons?
You are correct. There are certain non-class specific weapons that your monk is not allowed to use. A few such weapons are:
I do not believe there is ever any explicit reason in-game as to why the Monk cannot use them, however.