Polyamory is when a Sim has romantic relationships with more than one Sim at the same time, with consent from all parties.
The game doesn’t explicitly label relationships as “polyamorous,” but you can create them by tweaking your Sim’s Romantic Boundaries.
First, you need two Sims who are already in a romantic relationship.
You can set up a polyamorous relationship in CAS or build it up in Live Mode.
Option 1: CAS
Go to the Household Relationships menu.
Look at all the Sims in the household.
Make sure that each Sim who will be part of the polyamorous relationship is listed as romantically linked with the others.
Option 1: Live Mode
Flirt or do romantic interactions with the new Sim you want to include in your Sim’s current relationship.
Check your current partner’s romantic boundaries. If you have already disabled all the jealousy triggers in CAS under the Gender tab, your partner should be OK with open relationships.
If not, go to the Relationship tab, choose Change Relationship, and then Ask to Reconsider Romantic Boundaries.
You can only ask your partner to reconsider one romantic boundary at a time.
Just remember that your Sim’s Romantic Boundaries don’t automatically carry over to their partner or partners.
Once everybody is ready, the new Sim can officially become a partner to both Sims.
⇒ The key rule:
To set up a truly polyamorous dynamic in The Sims 4, all Sims involved must be comfortable with their partners being romantic with others.
This includes non-physical interactions like flirting or giving compliments, physical romantic actions like kissing or holding hands, and WooHoo.
And this is how you build a polyamorous relationship in The Sims 4.
Happy Simming!