Rudy also has low self-esteem, so they all match lolSylphy's low self-esteem in combination to putting Rudeus on a pedestal are author's way to make Rudeus' harem believable. In fact all 3 wives have level of low self-esteem and having too high of opinion of Rudeus that they resign to accept if Rudeus happens to marry multiple women (on top of nobles being known to be degen in this world). Order in which Rudeus marry also matters, if he married Eris first, she wouldn't let him marry another woman and she'd satisfy him sexually to the point of exhaustion most likely. Not sure about Roxy, maybe? Rifujin really put a lot of thought and care into the story.
It's understandable for Roxy and Eris because he literally saved their lives, while for Sylphie, Rudy is her first savior. This is also why Rudy thinks so highly of Roxy.
Later in the story we will see that Sylphie does not think that highly of Rudy anymore, but she still loves him very much; she just does not put him on a pedestal anymore, which is what Paul wanted to happen in the first place by separating them.
I'm very glad Eris is not the first (and only) wife, because she would not be able to handle what Rudy needs to do later in the story by herself.