I'm sure there'd have been a bit more to it if the story hadn't been cut to 3 volumes, but frankly I can't see anything but Yuu just bluntly telling anyone - including Touka - that she's dating Nana as the natural development anyway ^^;
Dialogue is stupid though. "Please don't tell anyone (that I'm dating this guy)" is what he assumes, so saying "alright, I won't, if you break up with him" is ... like, there's no point in not telling anyone if she broke up with him anymore. So ... yeah. Typical for a romance manga kinda thing but stupid nonetheless.