To expect that every person, real or fictional, is going to see flirting as just that with no intended meaning of "I want to go to bed with you" is in my mind more idealism than realistic. Everyone will respond to flirting to however they will take it, as being a real serious proposition, as just harmless conversation, or as something that will forever fly over their heads. Combining what I've seen in a Let's Player's playthrough of Dragon Age II with other people's, it seems that the producers have purposely made the three main and constant female companions of the game (Merrill, Aveline, and Isabela) as examples of what I'm talking about: Merrill seems to take Hawke's flirting as something serious, Isabela knows that Hawke is flirting with her, and Aveline during "The Long Road" (which is the only time that Hawke can try to flirt with her, though she is not romanceable) only has eyes for Donnic and will only question Hawke at the end if she succeeds in getting Donnic about whether they actually had anything between them.
Given that one of the Let's Players on YouTube had no idea what was going to happen in that situation and was trying to avoid either a serious commitment to Merrill or a major increase in rivarly, I liked how he handled the unwanted sex scene with his Hawke by having him respond to Merrill with humor rather than risk using the heartbreak response and making him look like a jerk who just used her for sex.
Fortunately that doesn't seem to be a problem for the male love interests that I had my female mage Hawke flirt with, as I had her flirt with both Fenris and Anders during Act I and only got serious with Anders in Act II with no unexpected visits from Fenris to create a dangerous love triangle.