I've never tried flirting with Samara and Morinth, so I was purely going on the 'Romance' page of the ME Wiki which lists both Samara & Morinth.
It gets tricky because there are characters who are open to flirting (Parasini, Sha'ira, Morinth, Allers, Vega, Javik), who obviously are not full romances. But then there are some that are kind of romances, but not really. Samara and Kelly all fall into that category. For me, I use the "Do they unlock the romance achievement?" criteria in any games where that's an option. I know it's not everyone's favorite method (particularly when it excludes their favorite 'romance'), but, to me, it's a clear indicator of who the devs consider to be 'real romances'. It's why I don't consider Samara or Morinth as 'romances' since the devs didn't code in those lines to unlock the 'romance completed' achievement (well, to be fair, I don't consider Morinth a romance at all because her romance literally ends with 'game over'. There's no way to successfully romance her without dying). But it's also why I changed my mind on Kelly. In ME2, I don't consider her a romance since it also didn't unlock the achievement. But it does in ME3, so I consider her to be a full romance for Shep.
It's trickier with older games without achievements. Like Juhani in KOTOR. I consider her a full romance, but I know some people on BSN don't. And there's really no good objective material around it either.