I didn't know he also snips if you tell you know already. That's so odd. Especially if you can ask him earlier of he and Felix were together. Why would he be surprised? lol!! I always go with the feigning ignorance response and just head canon that my IQ is trolling him to take his mind off his father for a moment. But I know Gaider put it in there for Dorian to have something to educate the player about. I don't even get why he's snippy if you tell you know already.
I wouldn't even know what the message is here: "Don't assume a gay man is gay, but if you don't know a gay man is gay, then you're stupid."
lol!
I'll say in advance that I don't know how this dialogue differs if you've already been flirting with him versus not. But on the occasions where I have picked the option to tell him that I already know, my guy -- previously flirted -- says, "That's not exactly news, Dorian," after which Dorian replies, "And why should it be? Why anyone should care, I have no idea," or something along those lines (my wording may be slightly off). I have NEVER heard that response as snippy toward my character, and his tone only seems frustrated at the situation, which is understandable.
But honestly, I find this complaint rather silly. They're not going to have a completely unique set of dialogue options and responses for every situation; friend versus not-friend, flirting versus not, etc. But they DO have some differentiation with some lines, while leaving others the same. The case of the Inquisitor's reveal that he also prefers men, and Dorian's varied response, is of the former, while asking him for clarification is an example of the latter. I really don't see a problem here.
(This is a general commentary, not to you specifically, Hazegurl.) Say what you will about "gaydar" or stereotypes or whatever, but there were people who went into DAI completely spoiler-free** that were surprised by the reveal that Dorian was gay. I don't know, maybe these are the better people for not being so narrow-minded as to focus on details like eyeliner and fashion sense. So no, I don't think that the expectation was that the player should be able to immediately pick up on the fact that he was gay.
The mere availability of the dialogue option to ask whether he and Felix were involved doesn't mean anything. You can also ask the same of Leliana in regard to Divine Justinia, or flirt with various NPCs as the opposite of their preference -- women with Cassandra and Dorian, men with Sera and Cullen. Before picking them, should I assume that Cassandra and Cullen are gay, or that Dorian and Sera are straight simply because those options are there? No. They are there for the player and as a part of role play. Take them or don't, the choice is yours to make and the NPC will respond according to their personality and preference.
** Not I, sadly.