Viewer Gender Confusion/Art

Nearly any modern portrayal of Mori Ranmaru falls heavily into this if you aren't already aware of the historical figure. Historical artwork of him is a bit better.
Late-20th-century and early-21st century artwork of Hindu gods or goddesses tends to portray them all as quite Bishonen. This is only partly true for historical depictions; while male figures were still drawn or sculpted as lithe, slender, and smooth-skinned, the secondary sexual characteristics of females were... much less ambiguous.
Renaissance- era depictions of The Beloved Disciple (John) and sometimes even John the Baptist (!) show beardless pretty boys who could be taken for girls. This is probably the reason for the well-known claim that May Magdalena appears in Leonardo's Last Supper.