"Monogamy" means different things to different people. You wouldn't think so, but it does.
Different people have different opinions on what qualifies as sex, what qualifies as flirting, what qualifies as emotional attachment... and thus, what qualifies as cheating. I feel a lot of relationship problems would be solved if we had a more expansive vocabulary in this respect, if people could state clearly at the outset and throughout the duration of a relationship exactly what they expect of each other.
As much as I've never identified with sexual jealousy, I've come to understand that some people are, naturally, wired that way... and, provided that both members of a relationship are on the same page about it, that that can be an okay thing. I frequently have to remind myself not to prude-shame.
There is a lot that Ellen and Nicole leave unsaid in this strip, which is normal between two friends who have had a close - and decidedly non-sexual - friendship for many years. They don't feel the need to rehash the nature of John's history, or Nicole's relationship with him, or why the threesome clause hasn't yet been mentioned to certain other individuals.
Perhaps they will later. Perhaps not.