Story.
I have no interest in seeing other people having sex, anymore than I am interested in seeing other people fight to the death. Cassandra's breasts and Liara's blue ass do nothing for me. But when two people are in a relationship, and they end up having sex, I as a member of the audience expect to see the climax (no pun intended) of that arc,
Key point: your use of the term "audience" here implies that you are viewing the media as a cinematic storytelling medium, like a movie or TV programme.
But we're talking about something marketed as a game.
and I see no more reason to hold back on the sex than there is reason to hold back on the battle against an antagonist. And sex is important, because most people don't engage in it with just anyone, and it serves to present the characters in a different light.
More important than all the other things they don't show? If it is that important, why do they have sex only once per game? It seems to me that some things are happening off-screen.
What about people whose characters would not engage in sex in the way the designers create the scene?
Some people are going to argue that this is a fighting game and etcetera, etcetera. But let's face it, virtually no one plays these games for the combat. There are much better shooters than ME out there. People play them for the story, the characters. And as such, I don't think I'm asking all that much when I hold them to the same standards I would if this were a novel or a movie.
My interest is the characters and the role-play. The more freedom I'm given to create my own story / narrative, the better my experience with the game.
You mention novel and movie here - both of which are vastly superior as storytelling mediums imho.