The Pale Parts (Breasts, Bum and Pussy) of the bikini Tan Lines.
The Objects of Sexual attraction are generally lighter then the tanned skin around them.
Making them "highlighted" and stand out.
The Girls in the Shower after Gym Class, teased Sara over her Tan lines, She just replied that they jealous of her Highlighted Goodies.
Guys would Irritate Web Cam girls over their Tan Lines, and she would respond that real Men pay to see her Highlighted Goodies