Then again, what did Tai think they were? Still fuck-buddies(excuse my language)...
There can be a phase in which two people are going on dates, getting to know each other, and are clearly interested in each other romantically and not just looking for a fuckbuddy... but they haven't yet reached the point of actually regarding themselves as a couple. It's the phase of exploring whether they
want to be a couple. (Some people's approach to dating skips this phase and goes from zero to "girlfriend" in one date.)
Dora started going out with Tai based on very tentative interest in someone she only knew as the "cute little redhead." It was probably important to her to linger in the exploratory phase for a while and make sure she really wanted to get serious with Tai. Now she seems to have decided that, yes, she really does want to... And didn't realize it might be helpful to mention that!
Dora may have thought it was clearly implied by all their snuggling in bed (and maybe more, but I dunno, we haven't seen any booty dances yet). I've had the impression that Dora generally reserves physical intimacy for serious relationships. Tai, on the other hand, is used to being around the Smif crowd where plenty of ladies are happy to fool around with her but not necessarily make any commitments, so Dora's readiness to get in bed wouldn't automatically translate in Tai's mind to "Oh, I guess she wants to be my girlfriend."