Sarah gets mad at Phil for a few things i.e. saying their relationship is non-linear, and preventing her from "dating."  They discuss living in different worlds, and the challenge of expressing different layers of  Sarah and Phil which could be called the "track stack" if you're a recording engineer.  So, how to live a domestic life, and express the other tracks.  The answer seems to be, make the leap and keep going cuz the grass isn't greener on the other side.
"There's still some mystery, even for us, about what we are," Sarah.