I wish sex was a necessary plot element in the story of my life...
Probably not the way I write it.
Or rather, when I think of sex as Necessary Plot Element, I think something like the following [ example from one of my WiPs. ]
Character A is a charismatic leader, somewhere between a political and a cult situation. Character B, our protagonist and viewpoint character, is a member of character A's group. Character B is loyal to this group on the grounds of shared ideals. Something happens that leads character A to misguidedly doubt character B's commitment. Character A sets circumstances up to end up in bed with character B on the grounds that this has previously worked to shore up personal attachment to character A's cause. Character B, while appreciating the interaction, feels somewhat disrespected, as character B's loyalty to the ideals of the organisation has never in fact been in doubt; this starts the process of disillusioning character B from that organisation.
That particular dynamic needs the sex scene in to work at the right levels. Whereas an entirely different bunch of characters in a different story who are running around together, figuring out the plot and staying ahead of the villains, might be having lots of nice friendly sex for purposes of mutual comfort, enjoyment and keeping their spirits up, but that's not to my mind something that needs to be onstage, they could equally well be playing Scrabble for the desired result on the story. [ Though probably collaborating to get cool words and/or high-scores rather than being competitively cut-throat. ]
I don't think I've ever written a sex scene. I'm pretty paranoid about those, because they always come out sounding all cheesy and soap-opera-ish. None of my stories really call for a sex scene, anyway, so I guess I don't have to worry about it.
They are distinctly difficult in the matter of vocabulary alone; if one doesn't want to be crude or clinical, or use indirect metaphors which can all too easily come out like tacky porn cliches, every sentence is a headache.
I think the "What's your story about" question puts a stitch in my craw because I don't think the layperson distinguishes "what happens" from "what's your story about." Or, in other words, "plot" from "story/theme."
Knowing what the theme of a story is before I finish it is another one of those things I don't really do.