Mikuriya closes the topic the same day it's brought up. It's because they can't ACTUALLY get married; the most they can do is adopt the other into their family to share their surname -- and Mikuriya doesn't want to do that. Both because part of what Kusaka wanted was to run away from his surname, and because Mikuriya wants to stand on equal footing to Kusaka.
I also don't remember if it ever says where they live, but there's a high chance the story either doesn't happen in prefecture that allows it OR happens before the Japanese partnership oath was a thing... and, the partnership program is also not the same as marriage. But it seems they'd go for it when/if legal marriage ever becomes an option.
it felt like there was something missing since the seme never brought up marriage after that.