It IS bad if you're not attracted to that gender AT ALL and it IS bad if you wish to have a love relationship with someone of your gender but you can't, ever. And I highly doubt you can hook up with whomever you want, you have to keep up an appearance after all.
I guess if you don't mind being trapped in a loveless relationship you're never able to get out of, playing your role of the loving wife/husband until the day you die, have sex with someone you hate to produce heirs, maybe fall for someone you can never have a relationship with ... by all means... it's not so bad.
I sincerely hope that at least for some nobles in Tevinter it works out for the better and they actually like who they have to marry.
Wow you took that post... very seriously 
Also just because you might be "generally" attracted to a certain gender doesn't mean having sex with someone you dislike, even if they are of your preferred gender, is not as bad. I mean personally I'd rather have to get it on with a nice female person than with a vile male person, and I'm 0% attracted to ladies.
Also it's not really a "relationship" ... I'm assuming your actual relationship would be with your lover? I mean if it's supposedly okay to have a lover?
And also I don't think you're expected to "pretend" to love one another? But I may be wrong about all of this.
After all this sort of thing was routinely practiced at one point in time; the wealthier you were, the less you had to pretend and the more "okay" it was to have lovers (sometimes even for women). Just, you know, don't produce any bastards because they have a nasty habit of later trying to kill you.
I think what maybe isn't clear is whether or not openly having a mistress or paramour is okay in upper crust Tevinter society or not.
Exactly, Dorian is an adult who can make adult decisions for himself, which he did. He left. He left Tevinter where he was unhappy. Where his parents were the selfish ones, forcing their agenda on him, disregarding everything he wants.
So did he really run away from an entire country?
That's also something I'm still not clear on (since I don't think it's explained in game). If that's true it really does make him seem very young. Which is okay; it's okay for characters to have flaws.
Generally speaking, yes. As long as you provide an heir and don't flaunt your affairs too much, nobles can carry on with others.
There's some social stigmas to avoid. Like Celene having an affair with Briala, an elf. Or Bastien having an affair with Vivienne, a mage. And married couples in Tevinter are expected to at least pretend to be happy with each other, even if they seek outside comfort.
Well Celene with Briala was clearly taboo, though I think because Briala was an elf, not a woman. I mean Celene wasn't even married and it was an issue. So it's clearly not an heir thing. But more of a social thing... a human lying with a disgusting elf.
Bastien and Vivienne ... ah, how much of a social stigma was that even? Didn't people know? The duchess knew and was totally fine with it. I can't imagine the rest of Orlesian court didn't know, the Game being what it is. In fact it seems to me that Bastien and Viv are clearly the actual couple. I'm not aware of any social stigma there, but I could be wrong.
And so that's true then? Married people in Tevinter are expected to pretend to be happy? Is that in WoT? And so I guess only straight people can marry in Tevinter? Or at least among upper class people?
It's pretty bad even if you are attracted to that gender and don't like the individual personally. Just ask Alistair if he does the Dark Ritual. And then pretend that everything is fine between you two? Dorian would hate that life even if he was straight. It's just not the kind of person he is. He's a man of passion. Being trapped in a loveless relationship would be hell for him regardless of the circumstances.
I had to do the ritual myself the first time; it was pretty revolting for me, especially since the game assumed my warden was totally into it 
And agree, he'd hate that life even if he were straight, or even if he were married off to a man, but that man was a vile human being whom he despised. Would be the same response from him, I imagine.
Which is why I feel like "the situation" isn't about sexuality, but this one scene, this poorly written scene, got away from its writer a little bit and really did make it about sexuality, perhaps because the subject is near and dear to the writer's heart. Which is understandable, but that's the point where you hope the other writers and editors step in and do a bit of tweaking.
Also, again, if all an upper class Tevinter person is expected to do is produce heirs with one's spouse, then, again, I wouldn't define that as "trapped in a loveless relationship." Because you're not really trapped if you can then have a cordial platonic relationship with the spouse once the babies are produced and can then engage in a loving relationship with a mistress or paramour.
It's just, back here on earth upperclass men were understood to be philanderers, so there was no such thing as "trapped in a loveless marriage" for them. It was really mostly the women. And gay men, of course; unless you were the king of England you still had to keep your boyfriends under wrap lol The society was just very sexist and homophobic.
So if Tevinter is the same way, then we'd have to say they're homophobic as well, and potentially sexist, unless both partners are held to the same standard (which actually would not make a great deal of sense).