To those saying that polygamy is bs y'all are making me laugh. My great great grandfather had 8 wives, each one's children becoming one separate branch of the greater family, this was an accepted practice and in fact it was seen as completely and utterly shameful if you had multiple wives and children that you could not provide for as a man.
More recent examples include, King Mswati III of eSwatini (formerly known as Swaziland) has 15 wives, while Jacob Zuma, former president of the Republic of South Africa has 4. Then we have the Mormons, who were forced to stop practicing polygamy although it is a part of their religious identity.
However, let's put aside polygamy irl because this is fictional tale of a world different to our own we should be talking about and note something critical that you missed. The setting of this story makes it extremely clear that polygamy is a thing and is accepted, and furthermore (and most importantly) Tetra and Almus have talked about it before and in this chapter that there should only be one other wife and that she should not break the family. It's Almus here who is conflicted due to his modern day Earth sensibilities, not Tetra since she was the one who laid out her terms clearly and with no room for misinterpretation.
Your feelings on it don't matter, if you're offended that she isn't acting like you want her to, that's a "you" problem, not a problem with the plot, as it's using the relationship between Yuria and Almus as the spark for a greater conflict regarding the politics of the region. As far as I see it, as long as I can read something and not have my "where the fuck did this come from Plot?!" senses tingling, it's all good, especially if it's something clearly different from Earth in an Isekai novel, after all, it's another world with whole new cultures, or at least that's what it is supposed be.