I thought the same, until I read the omegaverse explanation pages that this manga has. They're pretty different than the ones that we have in most omegaverses, in the sense that EVERYBODY can get pregnant, and omegas developed their pheromones and heat because of their low birth rate. In that case, it kinda makes sense that people would marry within their own hierarchies and that omegas would be ostracized, since they're a risk factor and nobody really needs them in order to reproduce. It also makes sense that unmated omegas would be scared of alphas. It's a different take on omegaverse for sure, but I can appreciate the diversity at least. They also said that suppressants had only been a thing since the 50s or so, so I'd be curious to know what omegas did to survive before they were allowed to have jobs, if they didn't marry alphas or betas. That's a bit of a worldbuilding plothole imo
This is the only Omegaverse BL Manga I've read where Alphas and Omegas marrying within their own subgender is not only expected but also really common.
I think it would make more sense if Alphas were crazy over Omegas and make them their wives or mistress one way or other. Doesn't seem unusual when the first group holds most of the wealth and power while the latter group is biologically in a difficult position to work and thrive.