I remember years ago on another discussion forum, I asked Dean McKendrick, who directed a lot of softcore films for Retromedia, how come they had a sex scene of 8-10 mins but we only got to see the full butt of Andrew Espinoza Long (the male lead) for literally 10 seconds. Dean said that it was really not intentional. They were not aware of that when filming it. So I suppose for a straight male director (and perhaps also the camera man), he just simply does not find the body of the male actors that sexy or erotic.
At that time, I made the point to Dean that male nudity was important in softcore films because there were lots of female and gay audiences. He did become conscientious about it and intentionally made sure to include some male butt shots in the later films. So if you watch the last batch of movies that he did for Retromedia around 2016, there were butt shots of pretty much all the actors involved. Unfortunately those were also the last batch of softcore movies with decent quality. After that, the whole genre was more or less dead.
Just to add one more point, Retromedia were making those films by the request from a network. According to the network, it was true that a lot of the audiences for softcore films were indeed female and gay. A little bit too late at that time. I can't help but think that perhaps many older softcore films could have included a lot more male nudity if they came to such realization much much earlier.