A secret relationship that basically only relies on sex.
A: Dude I heared you nailed Ashley yesterday!
B: Shhh, dude! It's a sexret!
"I had a dream about you"
"Tell me what it was about?"
"It's a sexret"
A scandalous secret involving sexual relations, usually pertaining to bands on tour.
Example one -
Girl one: I saw that guitarist (points to guitarist) getting it on with that girl over there (points to girl), and he has a girlfriend.
Girl two: I wonder if he will give us backstage passes if we threaten to tell his sexret.
Example two -
Alex and Lexy from Grey's Anatomy had sexrets.