Modern relationships in a nutshell.
This is why there are so many divorces happening nowadays, because one member (Or both) of the couple gets bored after some time and no longer sees the spark that made the other attractive, or not as much as they once found them to be. This can be for many reasons, not just sexual.
Then a third party comes in and gives the bored one what they're missing from their relationship (Attention, desire, excitement, etc) and then, you either get the cheating, or a seed of doubt about the relationship is planted.
As for this particular couple; I just think they found a common ground they both enjoy and keeps the spark alive.
It's high likely that all the dudes she fucks, they both find to be "prop dicks" for the fun of the moment, but never give a single shit about them besides that.
If they have truly managed to separate sex from love, then good for them. It is a really difficult thing to do, and even those who try it (Open relationships) end up failing after a while because of jealousy and feelings.