In the base game of The Sims 4, Sims are not able to have multiple romantic partners simultaneously. The game mechanics do not allow for polyamorous relationships by default. Sims can only have one official romantic partner at a time.
However, it's worth noting that with the release of expansion packs and game updates, additional relationship options and dynamics may have been introduced that allow for more open or non-monogamous relationships. These features can provide alternative relationship structures and allow Sims to engage in romantic interactions with multiple partners.
For example, the "Get Together" expansion pack introduced the concept of "clubs," where Sims can join or create social groups with specific interests and activities. Within these clubs, it is possible for Sims to engage in romantic interactions with multiple members. Additionally, there are mods created by the community that can enable more open relationship options and polyamory.
If you're interested in exploring non-monogamous relationships or polyamory in The Sims 4, I recommend checking out mods or custom content created by the Sims community or researching expansion packs and updates that might offer additional relationship features.