Or, heaven forbid, they are being used for their actual biological purpose of feeding a child. Then it doesn't matter how much is being covered, they are obscene
Nipples don't get perked when you're horny, they get perked by external influences like the cold or touch. They're designed to get perked when a baby wants to suck on them, it's not a sexual thing, that's just how our culture views them.
Just that those gets perked when you're hornedd....
hence they show out your sexual mood, hence a taboo..