What people interpret as a "sex" smell (in the case of Megan Fox in Jennifer's Body, it's Thai Food- see the clip above) is entirely up to the individual.
Here's a break down of the basics behind what a "sex" smell consists of:
Sex is essentially an exercise. When someone does a strenuous activity their body produces sweat in order to maintain an equilibrium temperature. When your body is emitting sweat due to the activity of sex, and your partner(s) body is emitting sweat, and you're rubbing up against each other--- that's more heat, more sweat, and thus, a component to a "sex" smell.
During the act of sex there is a release of a lot of different chemicals, and two (or more) people's body chemistry are mixing on the surface. One of the chemicals released is called a "sex pheromone" which is supposed to attract members of the opposite sex in order to reproduce. Human beings release this chemical during sex (since it is a reproductive act) in order to ensure attraction from the opposite sex prevails, so reproduction will continue within the species. Even if the act is with someone of the SAME sex, the body still reacts the same. Biology does not discriminate in this sense. The scent of pheromones isn't something you can actually "bottle", but subconsciously we can pick up on this scent. However, the scent tends to stay contained within the genital area.
3. Soap/Deodorant/Perfume/Landry Detergent/etc.
Whether the sex was planned and you spritzed yourself with a little Chanel No. 5, or you just got back from the gym and your deodorant is working at full strength, your body is still holding onto these external scents. During sex, mostly through the sweat, the odors from lotions, perfumes, or any other products you use on your body will be released and contribute to the unique "sex" smell being created.
4. Vaginal Discharge/Semen
While neither of these fluids are extremely strong, they do have a scent. Their scent (as well as their taste) varies from person to person depending on diet and body chemistry. Both of these fluids are released during sex... and are not always "contained" to one area, thus, spreading their scent--- or what little odor they do give off.
These are the major factors that create that "sex" smell. There are some minor factors as well like:
-The scent from the material of the condoms being used (since we should always practice safe sex)
-The scent from the lubrication being used in conjunction with the condoms
-Mood Candles, Incense, Room sprays, etc.
There you have it. An explanation of what goes into the smell of "sex".
It's unique to everyone, and can be interpreted in so many ways, like art...
Now isn't that a poetic concept.