You are correct and hit the issue well.
Ultimately, any arguments for interbreeding with Wevians are either
A: Pro-eugenic and anti-species (or, in other words, anti-racial). "The Wevians are really attractive and clearly winners, we should become them." This has been done historically, whether Japanese leadership hoping to trigger a breeding program to make themselves White, to classic cases of people trying to marry into the most recently prevalent caste (racial or otherwise), with even more comparisons done simply on an animal level: better species/races/breeds win.
B: Pro-choice and thus anti-species (or, in other words, anti-racial). "People should fall in love with whomever they like, and if it just so happens that the majority of people making such a decision will end their entire previous existence, community, and irrevocably alter their genome, losing what made them unique, then so be it."
Even as I say this, I would have a hard time not bagging the three alien waifus, but almost all of them are attractive in a human way...which then gets into Humanoid Alien theory.