Why are people weirded out about their parents being intimate

So my bf and I are on the way to his parents house. He called both and no one answered so I was like maybe they’re sleep or showering. He’s like I don’t want to think about them showering together. I reply well don’t but it’s normal for couples to do that. He’s all I don’t want to know about them showering or having sex. He said it as if sex is so gross. I’m like how the fuck do you think you got here? 😂

Why are people so grossed out by this ? Like sex is normal. His parents are very open like they’ll have dirty conversations at the dinner table.