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Superman has probably seen his parents have sex
(programming.dev)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. A showerthought should offer a unique perspective on an ordinary part of life.
The point was he has "X-Ray" vision, which lets him see through solid objects. Ergo, he likely couldn't help seeing them from time to time until he learned to control that power.