I am given to understand that in the books, Phryne is poly, whereas in the TV series it has come across more as a tendency toward casual sex. Having not read the books myself, I've no idea what that means for her dynamic with Jack -- but OH MY GOD YES visible from space, and they might have done something about it by now if it weren't for Aunt Cockblock Prudence.

I do have thinky thoughts about Phryne's promiscuity and also her flagrant disregard for the law in the context of that relationship; I feel like the show sometimes walks riiiiiiiight up to the edge of Phryne's free-spirited nature turning into a callous disregard for Jack and his feelings/professional standing/etc. But season three made me feel better about that issue than season two did, so I'm still happy overall.