(I don't consider the content of this post to contain spoilers, but if you like your gaming experiences pristine, better look away now.)
The single sex scene is brief and undetailed, although there is breast nudity in one version of the scene. The sex scene is triggered by the player making a series of choices about becoming more than friends with a colleague. If playing as a male character the scene can take place between him and a human woman or a humanoid female alien. If playing as a female character the scene can take place between her and a male human or a female humanoid alien.
So, if you are gay and male, tough luck. Weird.
Something like this has already been done by Origin Systems in Ultima VII: The Black Gate, which was released in 1992. That game allowed you to have more fun as a gay male. Of course, back then it was all a little less graphic:
The actual sex scene is some text on a black background.
It is ironic that the interactive medium has to fight to gain back the creative freedom it lost becoming (relatively) mainstream.
BioWare has included a relationship path in a large portion of its games to date. But, in keeping with nearly everything else in Mass Effect, the choice here is more detailed and has a lot more depth.
Jeez, I sure hope so. KOTOR was not very effective in this regard, not to me at least. I remember there being a certain area on the first planet where a trigger box in a certain place would always trigger a certain party member to make an idle remark in the hope you'd talk to him. The conversation itself seemed to be a kind of mini-game of 'repeatedly talk to gain his trust until he tells you about himself'. For me, it went more like: "Oh damn, the bridge, what do I do? If I cross it, the ugly and annoying man in my group will want to talk to me. Can I avoid the bridge? Can I get him killed somehow?"
However, despite my deep loathing for KOTOR, I must say I am intrigued by Mass Effect, sex scene or no sex scene.