Maybe I just missed your point but I don't think it's possible to really highlight that.
Many people think sex belongs only in the act and the visual medium is for losers.
Then there's people who think it's only valid if you limit it to the "missionary" scenes.
The onion peel goes on and on...
Even after you justify that, a good porn film as with any good film uses such techniques as fade to black to highlight and not censor a sex scene so we are back again to treating the effect as a tool.
Even without fade to black, a porn film will use such similar techniques to highlight sex scenes instead of take away from them:
-fade to white (often used by hentai films to signify post-orgasm)
-angles (link NSFW
As there's been a lot of discussion here recently about Hong Kong actresses and cinema, I thought I should do a post on the most famous erotic HK actress of all: Amy Yip. In spite of her global fame, there is surprisingly little concrete information I could find about about Amy, at least in English. Indeed, despite being famous for her obvious (enhanced) assets, even her nipples are a mystery: she always somehow manages to show all of her breasts, except the part everyone wants to see - a brilliant marketing strategy which has come to be known as the "Yip tease".
-underskirt and it's variation (viewpoint where the act is implied but the actual sexual organ is omitted)
-blanket over chest (a common technique used to not show nudity)
-Innuendo (besides the actual words, it could for example be a metaphor like a metallic drill as a penis for example)
-the "kiss" (i.e. a lesbian kissing scene depicting that something has took place)
...really in terms of highlighting and not highlighting, sex is no different than any other act in film. There's tons of ways to highlight, imply, satire, parody, take away it's sexuality depending on what you do with it.
A well respected artistic director like Kubrick for example can create a violent sex scene and justify it as a violent "art scene" even to the most anti-porn viewer. (The popular cliche of "it's a crucial element to pursue character development" or the other cliche where "the person who has sex gets it at the end and dies/humiliated" thus becoming more of a revenge film for anti-sex viewers.)
Then there's the previous quote about Cronenberg's Crash where the complaint was two-fold: 1) it was porn because it shows sex scenes and lots of it. 2) Since the correlation already has been made, then it's a bad porno because it didn't titillate.
...yet the entire movie judged as a whole is more about taking the fun out of sex scenes and making it more animalistic in order to un-arouse a viewer. (maybe even bore them to the actual act of sex they're viewing)