an illegal act
Immoral in the civil sense is an illegal act.
Could you sue someone for being non monogamous?
In a marriage where one partner cheats it is grounds for a civil divorce action that could affect the courts judgment regarding child custody and distribution of assets.
If it could be proved that both parties agreed to non-monogamy I donít think a court would consider a legal argument that the party being sued committed an illegal act.
In spite of their best attempts the religious community does not write the laws. Unacceptable is a more appropriate term than immoral. There are many legal rights that the religious community considers unacceptable.
We all have the right to determine for ourselves what is and what isnít acceptable unless the law prohibits us.
What we donít have the right to do is adjudicate and impose our values on someone else. That is illegal and immoral.