Hey there
Yeah I've also accidentally heard my parents getting at it and it bothered me for so long. For a few months I couldn't look at them the same and I felt sooo digusted and irked out. But eventually I realized that it's just a part of their romantic relationship, and that it's normal, and gradually I kind of forgot about the details of the incident and now it doesn't bother me...
But as this isn't a one-off occasion and you can hear them every night, I would honestly leave them a note saying something like "the walls are thin"...
Hope this helps!
Skye