The human mind lives in the gutter. It's true. If you're in the right mood, or maybe it's the wrong mood, anything and everything can appear to be filthy or related to sex. That sign pointing at just the right place. Or an advert juxtaposed next to something it shouldn't.
Or maybe it's just a perfectly innocent object, say a lid to an aerosol can, but in your mind, in most minds, hell in all our minds, it becomes something else. Something resembling genitals or a bum or some breasts.
That's just the way we're conditioned, sex is on our minds—or somewhere waiting to be on our minds— all the time. It's human nature, blame evolution for making us this way, for preoccupying us like this. Or better yet, just blame the internet for corrupting our innocence.
That way you can feel a whole lot better about having such a filthy brain.