An interesting title.
"Only into 2D", eh? I suppose that's as good an excuse as any to keep other girls from getting near him. Turns out this relationship of their, though, only started when he caught her masturbating at the office late one night.
That ending though. Ahh, the poor guy wants a proper relationship but was shot down immediately...But then she gave him the OK sign to try and get closer before taking that step forward for their relationship...And while he's super happy in the background, part of why she said so turns out to be just so that she can 'show those two' co-workers that she can nab herself a man, too.
Musashimaru-sensei used a tried-and-true formula: Sexy Onee-san Chief is tough-as-nails in workplace, horny-as-f_ck with her 'partner' (hang in there, Wakabayashi).