Brad Gorton in Get Real (1998)

51 minutes in the movie, Brad Gorton is naked on the bed. As he stands out of bed to put his underwear back on, we get a very good view at his butt.
There are also several scenes where he can be seen shirtless or in sportwear throughout the movie.

Paul Dawson, PJ DeBoy, Raphael Barker, Jay Brannan, Adam Hardman and Jan Hilmer in Shortbus (2006)

Probably the movie with most hard core sex out there. Guys with big dicks, hard-ons, blow jobs, penetration and even ejaculation! A must see with tons of nudity but also a nice story. Paul Dawson is sucking himself (quite a performance) and ends up cuming in his own mouth. You will even witness an unforgettable star-spangled-banner sung by PJ DeBoy while giving a rim job to Jay Brannan. Raphael Barker is only seen naked in straight sex action. Adam Hardman is seen in straight sex also but we see him very clearly jerking off his thick dick and cuming massively. His cum even lands on a wall! Jan Hilmer is an extra that we see having sex in the club.

Justin Long in Going The Distance (2010)

40 minutes in the movie, Justin Long is in a spa. He steps buck naked in a machine to get tanned. Although he covers his dick, we see his pubic hair and his butt. A few minutes later, he gets butt naked again to make love to Drew Barrymore. Justin Long has a nice bubble butt which has been oiled and waxed for the scene.

Antoine Cholet and Nicolas Gonzales in La commanderie (2010)

In Episode 8 of Season 1, we get brief nudity from Antoine Cholet and Nicolas Gonzales.
At the beginning of the episode, we get a very hot bath scene with Nicolas Gonzales but without full nudity.
Later in the episode, Antoine Cholet and Nicolas Gonzales are both naked one next to another. We clearly see both butts. We then briefly see Antoine Cholet uncut penis, and as they fight naked we can guess what appears to be Nicolas Gonzales bigger penis.

Thibault Vinçon in The Sentiment of the Flesh (2010)

Thibault Vinçon is naked in bed 16 minutes in the movie. A woman pulls the sheet away and reveals his very fine butt. 27 minutes in the movie, Thibault Vinçon is naked again on the bed. This time he is on his back and we can see his big uncut cock. 31 minutes in the movie, Thibault Vinçon is fighting naked. We see his butt and briefly his dick too.

Vincent Gallo in The Brown Bunny (2003)

In this movie, Chloë Sevigny performs oral sex on Chloë Sevigny. The scene is not faked. It's 3 minutes long, and we see every detail of Vincent Gallo's huge cut cock going in and out of Chloë Sevigny's mouth. His dick is so big , he can have a full hand holding it, while Chloë's mouth is already so full. The scene ends with Vincent Gallo cumming in Chloë's mouth.