Whoa, extensive report.
Then Martin Scorsese's Who's That Knocking at My Door from 1967 is the first mainstream movie with bare male.

She's also Gaby Hoffmann's mom
Gaby started out as a child actress in Field of Dreams, Uncle Buck, Sleepless in Seattle and The Man Without a Face.
Are you okay?

Louis Waldon having sex with Viva in Warhol's Blue Movie made around the same time.
Oh, the woman from Midnight Cowboy who looks like St. Vincent?
Tell you what…When I send my ex-wife her money…you can lick the stamps.

Muybridge's test footage included cinematic glimpses of naked men and women in motion way back in 1884…the footage is often used in documentaries.
Followed up with the 1899 French short film,
Le Coucher de la Mariee
Sergei Eisenstein's
Strike
(1925) has full frontal male workers showering together.
There were many nudist films made from the silent era onward, but hardcore porn existed as far back as the turn of the 20th century.
I seem to recall Horst Buchholz nude in some black & white German film from the 50s…he would later show how hung he was in a 1972 film,
The Savior
.
Warhol's factory films in the late 60s not only had full frontal male nudity, but also full erections…such as Joe Dallesandro, in
Flesh
(1968) and Louis Waldon having sex with Viva in Warhol's
Blue Movie
made around the same time.
I also find it a bit humorous that there's quite a number of all-male porn films playing in New York City during the late 1960s in theaters and they are advertised in the Village Voice along side the mainstream films. Those ads are available online.
I Am Curious (Yellow)
(1967) is noted for frontal nudity and sex, but more so due to Aristotle Onassis taking his wife, Jacqueline Kennedy Onassis (the former First Lady) to see it, and she walked out of the movie at mid-point. When a photographer snapped pics of her walking out of the theater, he asked her how she liked the film, and she gave him a karate chop sending him flying across the room.
Actor, Tom Baker (not the English Actor with the same name) is fully nude in Warhol's
I, A Man
from 1967.
And Joe Spencer is fully nude in Warhol's
Bike Boy
from the same year.
Harvey Keitel is full frontal in Martin Scorsese's
Who's That Knocking at My Door
from 1967
Rip Torn is briefly shown completely nude in the 1969 film,
Coming Apart
Are you okay?

Still taboo in modern times, too.

I love it when dudes dangle dork in movies.
You've seen Guy Standeven in something because the man was in everything.

that's 70s.
These guys were the first in 1960s - in the mainstream movie.
Art movies don't count.

try buster and billie 1974. jan michael vincent.