Explanation (смешанная)

Bucky joins the Avengers, and Steve has to explain to Bucky about all the various combinations of people who are sleeping together.
For this prompt at avengerkink:
So my favorite 'pairing' is Everyone is Poly Because Avengers. Usually it's written as a perfectly symmetrical love polyhedron where all the lines go to all the members and it's beautiful and I love it.
 
But it would be fun to see one where everyone is poly and in a relationship but not necessarily all with each other. Like Tony is in a relationship with Pepper and Bruce and sometimes has sex with Clint but he an Steve don't really like each other much. Steve is sleeping with Natasha and dating Sam. Natasha is disgustingly domestic with Clint and Phil and cuddly best friends with Tony, but she and Tony don't have sex. Etc, etc for everyone (including also people like Fury, or Agent May, or Betty Ross or whoever you like).
 
+Crack Bonus: The entire fic is just Steve trying to explain this relationship to Bucky.