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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.
He was too young to see this on his home planet. So yeah he might have seen his earth foster parents do it
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.