I really enjoyed that read. I was expecting it to go in an entirely different direction, and I'm really happy that it didn't.
My take on this is that sex and violence hold very different baggage in our lives. Most people will never experience the fantastical violence that pop culture shows us. It's all pure fantasy. In a weird way, showing violence to the point of gratuity makes it so cartoony that the majority of us have learned what is and is not real, so when a video game or a movie shows a scene of a guy's head exploding like a watermelon, we just brush it off as the cartoon that it is.
Sex, however, is harder to collectively get to that point, partly because despite making sex equally as gratuitous in our culture, the cartoony aspects aren't understood as cartoony. We still have this notion that the sex we see, while not real, is still attainable and can be something to strive for. I will never be in a situation where I need to go into bullet time to shoot a helicopter down, but I could reasonably talk to someone into sleeping with me.
And this is where video game sex makes me profoundly uncomfortable. My wife will roll her eyes at a violent game, but she will be visibly bothered if a sex scene comes on in that same game because, presumably, I am controlling the character that has decided to engage in the sex. If I'm trying to get into the game, that means on some level, I'm supposed to be into the sex as well, and that's where it doesn't sit well.
Arguably, a lot of this comes as a result of the limitations that you previously stated. For as far as video games have come, very little thought is given for real character relationships and the writing to actually support a realistic and mature sex scene. Mass Effect is rated Mature, but the sex is what middle schoolers believe sex is like: talk to the pretty girl and eventually she will sleep with you if you say the right things. If a game writes a husband and wife/dating couple/sexually romantic partnership/etc in a way that feels like humans, that changes the conversation considerably. Movies have proved they can do this, hence why we talk about actors' chemistry in their performance. When discussing video games, we get hung up on "who is the character I think is hottest/doesn't annoy my the most." It's all very shallow, and that sort of thing can get uncomfortable, boring, and sometimes dangerous.
In conclusion, good read.
