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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.
In the comics (at least those from my decades ago childhood), it's depicted as him seeing the same as normal people do - just through stuff. As such, I've always assumed it's not actually X-Rays proper & they just used that phrase to convey the concept of seeing through solid objects.