Wait a minute. When did they become sex friend, I’m confused

They've been for a long time, they were already sex friends in ch 1, I think. He said 4 years in this last chapter.

So they both get their thing up at the end of a good band performance/concert. They bumped into each other with boners after a performance and decided to try out sleeping together which ended well so they became sex buddies. This may not be accurate btw.