It may be partly connected with the fact that an R rated movie can have sex in it, but video game sex has been mostly a taboo subject (GTA as an early example), even for an R rated game. So it may be an example of the movement towards sex being included in games as it is in movies. Agreed about the immersion, but depends on how you “play” your character. Depending on the game, you can avoid the interactions you don’t feel fit with your character, but not every game.