
I don't understand why she has to be embarrassed about it. Why can't she just be comfortable? Even for the first time not all girls (or guys) get all embarrassed afterwards.

Right? I think they both lost their virginity right then though. She did have a close relationship with Sengoku, (like, close enough to continue taking baths in their teen years...) but I believe it was just a friendship kind of thing. Miyamura didn't act embarrassed either and he was an outcast. I think that they were just comfortable enough with each other to not be all nervous and stuff. They know they only like each other anyways.
well... what can I say? there are manga where the main characters take years to kiss (if they kiss...) and there is horimiya... in which they suddently have sex and suddently marry... and we don't even have the time to realize that it is happening...