Lets be honest here. When you are watching a violent movie or playing a violent game, you are there to experience the media. When you are watching porn, or playing a pornographic game (and I mean a game where the explicit sex scenes are the point, not softcore throwaway stuff in something like the Witcher), you are there to jerk off or spice up sex between you and a partner. Porn is a means to an end. Even if they have great narratives, porn games are always going to have a hard time rising above smut in the eyes of most people, since the majority of users are playing to arouse themselves/get themselves off. People are uncomfortable around explicit sex and porn when it's public because sex and personal sexual desires are private things (for most people). Most of us enjoy pornographic material, but I doubt many people would want to go through their porn collection with their mothers. In fact, most of us like to keep our porn habits neatly tucked away until we have need of them.
Violence is not the same, and lacks the privacy aspect that surrounds sex. Equating sex and violence doesn't really work since sex is personal and violence is not. It doesn't matter how many arguments you make about sex being more positive than violence, a lot of people don't want reminders of pornography in their life outside of the occasions they choose to seek it out in their bedrooms. I know that Europe is more liberal with non-sexual nudity, but I don't remember hearing about hardcore sex being mainstream fare that is shown on daytime television, and projected in (non-adult) theatres next to showings of the Expendables 2 and The Dark Knight Rises over there.
I have no doubt that having pornography on Steam would turn off more people than it would bring in, and I am sure it's not entirely a US thing. Video games are already fighting with the same stigma that comics have in that the medium was heavily aimed at children and teenagers in its early years. People are just starting to except soft-R levels of sexuality in their games. Littering the largest digital store with pornography will do more harm than good in expanding the acceptance of videogames as mature media. Steam has the right to look out for their self interests, even if it is at the expense of some indie games getting greater recognition.