"It smells like sex in here," said every woman ever after a particularly dirty romp in bed. But when you sniff out sex, what exactly are you smelling?

Well, to create le parfum de knocking boots, you take one part semen, one part vaginal lubrication and secretions, and combine them between two sweaty bodies, says Sara Gottfried, M.D., author of The Hormone Reset Diet.

But since semen is alkaline (it has a high pH) and anything that comes out of your vagina is acidic (low pH), the two don’t combine to smell like something that’s a little semen-y and a little vagina-y. When a base and acid combine, they react to each other to create a goo with totally new chemical properties, she says.

The exact smell you get after each sexcapade largely depends on both of your down-there pH levels. Douching, frequently washing your vulva with soap, and sitting in bubble baths can all throw off yours, says Darius A. Paduch, M.D., Ph.D., director of sexual health at Weill Cornell Medical College. Even the food you eat can alter your down-there pH.

Using a condom? You can still expect some smell. Having sex—particularly if it’s rough or he’s well endowed—can cause microabrasions in your vagina, which in turn lead to inflammation-caused smells, says Paduch. Meanwhile, sweat, spermicides, and lubes all give impart a certain musk.  

Still, the smell shouldn’t be “bad”—and it definitely shouldn't be fishy. If it is, you should visit your doc to make sure you don’t have a down-there bacterial imbalance like bacterial vaginosis that’s throwing off your pH, says Gottfried. He might want to get checked out, too: Infections in his semen could also be to blame, says Paduch.