Yeah, I know what you're thinking. Who wouldn't want to see that? Particularly if said sex involves hotties we really like. But let's think about this for a minute. Ever since sex started to creep into cinemas 40 or 50 years ago, there has been an unspoken yet mostly obeyed line in the sand between where films can and can't go with their sexual content. There are practical reasons for that line. There are also lingering ideas about what is appropriate to show and what isn't. Still, in the 21st century, are we ready to erase that line and accept some authentic sex? Would that really make a better film?
We all have our favorite sex scenes. Whether they feature stars we're particularly hot for, or show off generous amounts of skin, many of us have that one movie, or perhaps several movies, that make us drop the remote when we find them playing on cable. Most of us enjoy such sights, whether we admit it or not. Many eagerly anticipate that part of the movie or TV show when the hotties start to take it off and get down to business. So one could say that we're already sold on the idea of celebs having sex for us. However, despite any general openness to the idea we might have, society still imposes its barriers on how such things take place. First and foremost, we in the States have a ratings system. Other countries also have them. These ratings systems tend to be quite unrelenting when it comes to sex scenes. Basically, getting down to a certain degree of friskiness on screen gets your movie slapped with an NC-17 rating, which is pretty much the kiss of death for any movie, as theaters generally refuse to show NC-17 films.
There are also the lingering moral issues from when Hollywood was ruled by the Motion Picture Production Code, which were a strict set of regulations that limited the content of motion pictures made between 1930 and 1968 when the code had authority over Hollywood. This is why most of the films you see in that span feature no cuss words and little if any sexuality. That system is long dead, but the moral objections it represented still weigh heavily on the acting community. Basically, no one wants to alienate their audience by taking it too far. Despite the lack of any established moral code and the claim that "any publicity is good publicity," there does tend to be an unspoken code which makes many actors view explicit sex as simply not worth it.
Despite all those limitations, or perhaps even because of them, some actors in the last 30 years or so have started to open up to the idea of non-simulated sex scenes in an attempt to portray something genuine in the characters they play. At first, these performances were merely rumors, like with the infamous scene between Donald Sutherland and Julie Christie in the 1973 thriller DON'T LOOK NOW. Rumors from the set were that there was nothing simulated about the sex these two have in their scene together (you can view that highly NSFW scene here).
Later, more highly publicized sex scenes in films like THE BROWN BUNNY and ANTICHRIST moved away from rumor and into clear and obvious real sex. What those scenes did for the movies in which they appeared, or the actors that took part in them, is debatable. For some films, the attention they garnered by having sex for real was the only reason anyone remembers them, or the performances the actors gave. For other films, such sex was was something integral to the characters and the film. So the motivation of the film maker and the actor actually performing the act is a big part of whether real sex should even be considered.
However, one also has to consider basic human psychology and how we as human beings tend to put people into categories from which it can be quite difficult for them to escape. Just like how action stars find it difficult to crossover into historical dramas, there's a reason why your average porn star doesn't often break into the mainstream cinema. That's because when you're known for having sex, you quickly find yourself pigeon holed into a specific area of film making. So an actor might become the courageous breakthrough artist who brought real sex to the multiplexes, but that might not stop people from labeling them a pseudo-porn player who debases her or himself with real sex depictions. That could lead to horrible typecasting, or worse, no roles to play at all, which is the nightmare scenario for any actor.
The basic idea then is necessity. Do we need to make true sex a staple of film making? Aren't there certain sex acts better off implied than faithfully performed? Would Brando violating women with dairy products for real have added anything to LAST TANGO IN PARIS? Not really. Would Monica Bellucci's excruciating rape scene in IRREVERSIBLE be any more horrifying in its stark depiction if it wasn't just acting? I don't think so. Stuff like that is already intense enough as it is. I suppose if you value the idea of eliminating all remaining boundaries to an actor's ability to tell an authentic story, then true sex might do something to help along some actors' ability to own their part. However, many other actors would probably be more content to simply act like they're having sex.
And that gets down to a basic issue here. Actors don't need to go into space to play an astronaut, or travel back in time to play Abraham Lincoln. That's why they call them actors - they act. So they really don't need to participate in actual penetration to achieve authentic intimacy. No matter how method an actor might become for their art, there's always a clearly delineated line between the character and the person playing them. Sex is about the most real act one can engage in, which is why most actors seem content to keep actual sex strictly for themselves and their significant other, so they can go home at the end of the day and still feel like they've left the movie on the set.
So would I say that real sex is crucial to making a good love scene? No, not really. Depending on the movie, it could work or it could be a distraction. What really matters is in what context it's used and how freely the movie industry and the public at large accept people's right to engage in and view this material. As long as morally superior institutions, both official and unofficial, want to make our decisions for us, such intimate scenes will remain where they are now - on the periphery of cinema's more avant-garde outer reaches. If we can grow some maturity and allow people to see and do what they like, real sex in the movies might shed some of its taboo.