Well, in that world marriage can be just as well considered obsolete seeing how everyone seems to do it with anyone. Look, don't get me wrong, I like the art, but this is one of the few manga series that make me hate every single character in it.
Its a fine hentai for the most part but lord it takes its sweet time for the mother and son to get together. Absolutely no reason why this needed to be dragged out 6 volumes and then get only like 2~3 chapters of the main couple fucking, you know the mother/son angle that pretty much everyone is reading this for? Thankfully the author made plenty of bonus chapters of the two of them to make up for it.