Thank you for your thoughtful comment!
I walked in on my uncle watching porn once when I was 5 or 6, and it wasn’t a traumatic experience. I only remember it because of how the adults around me reacted when I said something.
This reminded of the parenting advice that when children hurt themselves (e.g trip over) freaking out about it is training them to freak out at the pain more - the calmer you are about it, the less tears are shed.
Basically this is the same way I feel about nudity - you don't freak out and yell at your kid for accidentally seeing you naked, you instead explain that people appreciate privacy regarding their body, so one should look away when someone is undressed. I presume this is what you mean by 'the way we treat nudity and sex isn't great.'
Ethics aside, there could be legal ramifications
Yes, I actually found out recently that there's a crime called corrupting a minor, which usually means showing them porn or sex acts. I think it would only apply to overheard-sex if the child made the parents aware, asked them to avoid the activity, and yet it continued. I, not being bothered by it, simply never bought it up (pretty sure I was a teenager when i first overheard.)