I live in an old terrace. I hear bits of noise from both sides normal life stuff, not an issue. Other neighbors play instruments and I can hear but it's no problem. Both sides have kids at least some of the time too.
This afternoon I could hear my neighbour (non music ones) having really loud sex. It's a fella who lives there I've had a few conversations with him he's in his 40s and lives alone. He's nice enough and no bother as a neighbour. It was the woman I could hear moaning. Tbh it sounded really fake and porny but whatever, they might like that. But that did make it more obvious and a bit cringe.
I put a note through the door saying we could hear and perhaps put on some music. It was polite in tone. I thought this was better than saying it face to face! But now I'm wondering if I shouldn't have done it. There's nothing I can do now it's done! But people are entitled to have sex in their house. I'm super sex positive but I do think it's a private thing and not something kids should hear for example. We have family over a lot and I'd be really embarrassed if this happened when they were over. I do massively over think so, a genuine wabu.