The person using this term ignores the fact that tan lines often look ridiculous and embarrassing for women who have them, because his purpose is to show how manly he is.
For these types of people, tan lines = woman sunbathing = swimsuit = sex. Therefore, an ostentatious display of heterosexuality must occur, in which love of tan lines is boasted about as loud as possible. This is in keeping with this type of male's equating ANYTHING female with wanting to 'fuck' said thing.
This technique ensures that no one questions this type of male's manliness, although he is often hiding an inferiority complex or his homosexuality.
Passerby: "Why? They kind of look ridiculous and embarrassing...?"
White male: "Tan lines!!! Fuck yeah!! I get laid A LOT!!!"
Passerby: "But you didn't answer my question...?"
White male: "Yeah!!! Tan lines!!! FUCK!!! Pass me another beer!!!"