I always pick the wrong lines and never get him flirting with me. *pout* I only ever realize this well after the fact. I wonder how old he's finally supposed to be in DA:I and if we'll see him. The years have always been kind to him. Ahem. 
The only female warden I had who didn't have the flirt with him was a CE, and that was because I chose the wrong dialogue (mentioned Nelaros), and I didn't feel like reloading to fix it.
Back to the topic at hand, I adored Teagan saying that your mage Warden would be worth the risk of marrying a mage.
I never really intentionally flirted with two characters in either game,
Yes! I loved that, though I took it as gallantry (especially with the elven mage). I particularly like Teagan's VA since I've run into him in other games, and he always manages to evoke some strong reaction (usually he's someone I detest, which makes my Teagan obsession even funnier).
I often found it hard not to accidentally flirt in DAO; I started checking affection status every time I talked to a LI I didn't want to romance.