Actually, a significant portion of men don't like to watch men in porn, or in porn games. So, they only watch lesbian porn, and complain if a man is shown during a sex scene in a game. In addition to immersion in the MC's role, it is a principal reason that many game developers severely limit what is shown of the male anatomy during their sex scenes.
I think it's silly, and leads to a lot of disembodied penises and even tongues. It also ignores the fact that there are any number of sexual positions that emphasize the female's body. But, that's just my opinion, and doesn't alter the fact that it happens... or why.