Probably not what you're looking for, but the description reminded me of 'The Family Cuckold' which was posted on ASSTR.
I'm trying to locate a particular obscure story in one of the old forums.
It involves a cuckold teenage son watching his parents spoil his teenage best friend who becomes their bull. The father takes on the primary cuckold role by being with his wife and the friend a lot, while the son gets excluded and is forced to spy and masturbate.
There is a scene where he watches the three of them flirt in his family pool. And another where he watches the three of them through the crack of the door of the master bedroom, seeing the silhouette of his best friends on top of his mothers, seeing the covers ascend and descend between her spread legs, as his father masturbates at the side of the bed.
This story was just written randomly on the forum (not on other sites ie" literotica etc) by some unknown writter and I was very impressed. Wondering if anyone could help me find it?