You know I typically have reservations about these sorts of developments for a few reasons already raised. (It feels like a cop out for indecisiveness or doesn't permit proper heartbreak or rejection etc.)
My main concern here is If Rin and Kurohime agree to this, then the temptation is to slot everyone into a Toudou harem, and that is not the ending that is justified by either Rings commitment, or (I would say especially) Kurohime's feelings.
Our girl has had at least 50 chapters of development to get here, she deserves some exclusivity, and I would hope that make it a true, healthy poly relationship the next few chapters after they agree to this are focused on Rin and Kurohime exploring each other and their feelings for each other independent of Toudou. I liked their interactions in the dodgeball arc, and it'd be nice to see that development taken further.
I do have confidence that the authors will pull this off though. Having turned a dubious concept into one of the best thought out and portrayed examples of love in all its messy glory in modern manga.
(Also for those calling for Yanagi to be added, what the hell man she has the vice prez right there!)