There certainly has been a lot of sex in games, particularly in the last few years. Whether this is meant to shock, excite, attract, push boundaries, keep the medium fresh, there certainly has been a fair bit. But is it mandatory?
Mass Effect 2, or so I've read numerous times, has a sex scene. Depending on the character's interactions with the other characters in the game, I'm assuming Shepherd forms a relationship and eventually has sex with someone. This may seem a reasonable aside to the plot or even part of 'the story' that the player is telling, if they/their Shepherd feels strongly enough towards a character. If you're not interested in that, I'm presuming it doesn't happen, as even going down a relationship path, there's still choice in the dialogue and you can probably take it as far as you want. If it didn't have this choice, and was a scripted event, then that may turn players off to the story that they were previously enjoying, or many other possible reactions.
Dragon Age Origins also has the option for it, but I'm sure that, again, it's not "scripted" and if it happens, you've pretty much set out with that intent, and there is also the possibility to carry on the game without ever knowing it was there.
Heavy Rain also apparently has a sex scene. I don't know anything about this one either to be fair, but with the nature of this game it seems to be "sex scene coming up do you want it to happen, yes or no" almost like the option with No Russian; as opposed to the joke with the false interactivity in the cinema in
Futurama, and opposed to the aforementioned western RPGs take on it, with dialogue trees and actual direction.
Just imagine if
Halo, for example, had a sex scene. There is no way that could possibly work. Moving on.
The only game I can think of that had sex in and that I played, was
Edmund. I played this a few months ago after hearing someone on a podcast (most likely Anthony) mention it. Being free and having time to kill, I decided to try it out. Knowing what it was about going into it gave me a certain impression of the game, but I (accidentally) started on the jungle story first, which was strange and seemed a dumbed-down megaman clone as opposed to something about sex; with things waiting to kill you and checkpoints etc. and this sort of "gave a reason" for what would happen in the taxi part. I by no means condone the character's actions, and I'm not sure whether the sex was mandatory at this bit or whether you could kill the victim instead of putting them through that.
When I played the taxi part however, I shot the rapist as soon as I got in the room, instinctively, and it was game over. I retried, with continuing as the narrative would want me to, hearing what he had to say, and I shot him again, knowing full well what the result would be. I can't remember if I tried to walk away, but if the game had just been about rape and that was it, with no reason whatsoever and no alternative outcomes, then that linearity would have reduced player input to essentially pressing right and maybe jump a few times. That sounds odd.
Basically, games by their nature reflect the desires, needs, wants, etc. of the players, and this interaction (among other things) makes them games as opposed to films. Whilst sex doesn't fit into the narrative in every film it's portrayed in, it's generally more accepted as there's nothing we can do about it. In games, we SHOULD have the choice if we want to not do it or not, otherwise the game will remove all interaction, say "look I put sex in this game, look at me", even if it's not integral to the player's choice, story, or interests. Making the player have sex when they don't want to? That's practically rape.
LOOK WHO CAME: