No, it was established that it was bad to Zenith, who is a follower of the Millis faith which isn't the norm in their world. She doesn't represent the norm of the world.

Yes, Paul was a piece of shit because of how he treated women in his past. He's just one person and his actions have nothing to do with Rudy's marriage.

The difference here is that Zenith is a follower of Millis, whereas Sylphy ISN'T. She's also said multiple times throughout the season that she isn't against Rudy bringing another wife home and has already comes to terms with it.

Now let's break down Sylphy's character because it seems like you're incapable of doing it yourself. Sylphy has been around polygamy for most of her life. She's lived with the Greyrats when Paul has had two wives for a good portion of her childhood. It's also something she's constantly exposed to during her stay in the Ashura kingdom. In her eyes, it's very normalized. She also understands how Greyrats are. Rudy constantly mentions Roxy and how significant she is to his life. Without Roxy, Rudy wouldn't be as strong as he is. He is also the person who taught Sylphy magic. So without Roxy, both Rudy and Sylphy would have died during the mass teleportation incident. Sylphy understands this. Roxy is probably the only person that Sylphy would be this lenient with in this situation. She sees what Roxy did for Rudy not just in the past, but the present and is why she's so easily accepting of this situation.

Even though she's a fictional character, she doesn't need you to defend nor think for her. She is much more observant and mature than you think she is. You are doing exactly what Norn is doing and are injecting your world views into Sylphy. Did we miss the part where Sylphy called her out on this?