I'm no expert on anime, but the two big examples I'm familiar with, Sword Art Online and Demon Slayer both feature many masculine adult characters[1], but usually in a mentor or antagonist role. The protagonists are literally boys, who by definition are less masculine than grown men.

[1] The Legendary Hero Bercouli in SAO and Sakonji Urokodaki in Demon Slayer are two that immediately spring to mind.