If the long term relationship reaches -50, they will become enemies.
The enemy status remains even if they talk on phone and it goes down, but if there are transactions in person (even talking at a dining table) the Sims can cease being enemies. Popular Sims fear becoming enemies with a friend.
It is all about your own interpretation about what is going on - sometimes when you are playing, you can miss interactions which affects relationships because you are micro-managing another Sim at the time.
@BoergeAarg61
I think my interpretation is more likely - just like in real life, when a family member is carrying out their ablutions, someone else wants to as well. :)