Partner's Abusive Partner
If your partner has an abusive partner, then YOU are in an abusive relationship also, because a polyamorous relationship is a relationship between all the people in the respective relationships.
When a father comes home and beats his wife in front of his children, those children are being abused whether or not he's hitting them. Seeing someone you love get abused hurts you, and so you share the abuse.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker