Ever wonder why it typically takes three dates to get it? It's because the road to sex is a narrative unto itself, and thusly incorporates the traditional three act structure of storytelling:
We start with our opening. On a FIRST DATE we establish the main characters of the story. Most productions of "Sex" have only two characters, unless we're experiencing a popular offshoot called THE LOVE TRIANGLE, or the indie-alternative ORGY.
In establishing our characters the basic reconnaissance should reveal things like: name, age, what you do for fun, what you do for boredom, favorite books/movies/music, positions on celebrity/pop culture, are you smarter than a 5th grader, etc. Once the information has been received for processing, the characters will now begin to evaluate their feelings about each other based on this basic outline.
At some point during the opening someone needs to do something out of sort. In screenplay writing this is called the INCITING INCIDENT. It leads to the second act. If you don't get this inciting incident you don't have a story. Instead you have a sucky first date and no follow-up. Either character can spark the inciting incident, but traditionally the male character will do something that puts himself on the line. The cliché (and TAN favorite!) means of doing this is to make some sexually provocative remark. This comment will necessitate a prompt response from the other character creating an incident or dialogue that leads us into the second act.
We now have an inciting incident, and with the basic outline of the characters in place we can now establish a DYNAMIC STORYLINE. An original phrase I just concocted, the dynamic storyline begins when both characters have finished PRELIMINARY JUDGINGS and established the balance of power. This is usually determined soon after the inciting incident, i.e. the guy stumbles and now the girl is in command, or the guy nails it and now the female is playing catch-up. The bulk of the second act is created by this balance of power swinging back-and-forth. The more swings, the more dynamic, the better the story, the more compelling the relationship. This all has to be done within the confines of the characters we've established (i.e. that nerdy pansy-ass white guy can't up and change into a cool assimilated negro on a whim) The second act can be of varying lengths, but it usually represents the bulk of the narrative.
The third act begins when you have an incident more compelling than the first, that creates a MOMENT OF CHOICE. The choice is whether you’ll be proceeding to sex, or determining the story thus far has been insufficiently entertaining and to be abandoned. Anyone can instigate this moment, but whoever sparks it determines the balance of power leading into the GRAND FINALE.
At this point the story has either been abandoned or will close with EPIC SEX SCENE. This grand finale will either disappoint or have everyone eager to see what happens in the sequel. The sequels can take the form of any genre. You may get a XXX story, where the characters just have sex all the time. Or you may end up with a nice romantic comedy, with dates and love and emotions rising and falling. Those sequels usually suck, but sometimes you hit upon a classic. XXX movies, on the other hand, don't have the same upside, but at least you know you can watch it and get off.