I completely agree on this, I believe the writers wanted to prove that Monica and Ross had a very close relationship and apart from siblings they were also friends but they were far too open about their sex lives in front of each other, much more than I would be in front of any of my female (or even male) friends never mind my sister.
I think they realized about this and it seemed to improve in later seasons, I think the penny dropped after having Monica get excited imagining her brother having sex in the TOWT free porn as early on in the next season they seemed to lampshade this with Danny and his sister, after that things seem to be more normal, with things like Ross freaking out when he sees Monica and Chandler kissing or being made unconformable when Monica bursts into the room asking Chandler to have sex with him when she's trying to conceive.
However, they still had incest jokes played for laughs, topping them with Ross mistakenly kissing fat Monica, which was come to think of it an act of genius on the writers part considering that the people who like incest jokes would probably also find fat shaming funny.