Well, that could have gone better... On one hand, Ai missed her chance, and on the other hand, Nagisa got a bit overwhelmed there and got a date before he could properly express what he wanted to say - which did seem like he did want to turn her down.
The thing is, a romance like this needs a bit of impetus to move it forward. Much like older-school yuri, the assumption that romance isn't on the table leads the characters to ignore their budding romantic feelings. The appearance of a rival sparking dissatisfaction and the question "Why am I dissatisfied?" is a very common way to push things along, because it has so many handy ingredients.
Oh, you're not wrong. One day they might even agree this helped them move forward faster than it otherwise would have gone. But right now with their plans derailed like that, it's certainly not what they would have wanted at this time.
