My best friend played Dragon Age Inquisition unspoiled. She did the Mage quest early and then goofed around at Haven forever because She Wanted To Totally Finish the Hitherlands (I was like 'Go seal the breach! Get your last party member!' and she kept ignoring me and yelling at me not to spoil her). This, of course, led to a lot of time with Dorian before his personal quest, and opportunities to flirt and be playful with him.
So, look, I can understand: when she finally got around to his personal quest and the 'I like men' deal came out (which I had been spoiled on, but probably would have guessed anyway, honestly), she actually called me up wailing about how I hadn't told her (I totally did a 'But I thought you didn't want me to spoil you? :D' routine, which was probably mean of me). You can definitely flirt with Dorian as a female, and while your first flirt as a guy with Sera ends with a 'Pfft! Not even!' I could definitely see how people could take that to be a 'not now' (just like people were confused by Viv).
However... if you're keeping up with them in conversations and the War Table quests involving them (and if you're interested in them, you probably are), both Dorian and Sera's quest take place pretty much right after Skyhold. Both will turn down the people they're not interested in then firmly. There should be plenty of time to switch over to someone interested in you then.
(Actually, the person I have interest with is sadly Cass (who is still one of my favorite characters and romances despite it, mind)- a female can flirt with her constantly throughout the game and think they're on her romance path, and it's not till after her personal quest (which tends to pop up more midgame-y) that she turns a female down. That can be far after the point you can jump on another romance, save for maybe Cullen. Definitely past the time you can move to Josie or Sera, if you're interested in pursuing a female.)
So, I mean... my sympathies, but if you keep up with her, Sera should tell you her preferences pretty early, and she *does* turn down a male character straight in the very first flirt.
The whole 'I should be able to make a lesbian like men by showing her enough devotion' sort of has a creepy tone to it, though. No, the character is not real: but gals who like gals are very much real and in this fandom (and on these boards), and many of us have dealt with enough of that sort of toxicity in real life already. No one is a slot machine: you do not put coins of niceness in till the sex falls out. The less video game characters fall in to that trope the better; I think almost everyone has wanted characters who are more like real people and less like 'Yes, I never take you out in the party, Morg, because you disapprove of everything I do, and when I do you hate me, but HEY SHINY THINGS! Let's fall in love!'.
As that was kind of dumb.