"But a portrait of a man standing naked is obscene"
Yes, unless that naked man is wearing a prosthetic dick in series like "Spartacus" or "Game of Thrones". Then its "OK".
Seriously though, I do think it's more of a "You can't show dick" issue, than it is a "male nudity issue" per se. Various forms of visual media over the years (film, television, video games) have shown nude males from the backside or from angles (breathy sigh for Nathan Fillion in Firefly). But yeah, it is "weird".