I bet Iroha and Izumi starts going out at some point and that's what makes the main pair think "oh, shit, we can be like that too".
Probably not? I mean, Nanaki is aware that girls can have romantic feelings for each other, because, you know, Izumi did confess to her and everything, and she made it pretty explicit that she meant love in that kind of way. So they don't need to break the "oh, two girls can date each other" barrier.