Im not even sure what your talking about.. this has nothing to do with love or couples or marriage.. I'm going to make it very simple where im coming from. Alistair is the last of his family, Ferelden was unified by his ancestor Calenhad, This bloodline is a symbol to the people of FERELDEN! It is VERY important to countrymen to see the forefathers legacy continue in some form or fashion, think modern day British Royal family, sure they don't have the same power they used to, but it is a symbol of pride for the people of that country that they maintain the royal bloodline, and not let something like that simply disappear. The same could be said of Ferelden, the people are obviously very patriotic, and passionate about their homeland, and it's rulers. I will again direct you to Arl Eamon's blurb about having a Therein on the throne. It Matters to the nobility, and thus indirectly to the commoners who owe fealty to the nobility who owe fealty to the royals. It doesn't matter one lick on whether or not Alistair loves Anora, the Warden, or if the Warden loved Anora. You are putting the political and by association the stability of ALL OF FERELDEN in jeopardy if there is NO HEIR to ascend the throne. Orlai or any neighboring kingdom could easily Sack Ferelden because there is no ruler, no unifier to bring the nobles and their armies under one banner to prevent it, not to mention Civil War would break out as each noble family would obviously fight to take the throne.
So your point of love, and marriage, and whatever else is unimportant, this is for the stability of the future of Ferelden.
In that case, why make such a big fuss over why you couldn't have kids with him? Considering how you wrote your initial post, you were less worried about having or not having an heir and more worried about having kids as a Warden/Anora pairing Because Happy Ending. And that's fine, but it also ignores the wishes of plenty of other players who'd rather not have their characters forced to be pregnant. Believe it or not, that is a thing.
The having an heir thing doesn't bother me, as a real-world person not living in Thedas. I expect it to lead into an interesting story in the future. And that is a very excellent reason why there isn't/won't be an heir, unless Alistair or Anora, or whoever is in charge, names one, or there's a bastard child somewhere. If I were a fictional character in Thedas, it would certainly bother me, because the plot events would look pretty dangerous to me.
We don't "deserve" to have an heir or children from DAO. We did nothing but buy and play a game. A happy ending (assuming we even have that option) doesn't always have to involve children, and I repeat, we don't know that this isn't part of some grander plan for a future game plot.
I'll throw this out there, taint, infertility, probability i don't believe any of that matters, if the writers say its possible then it's possible even if it's one in a million. Will they write a heir for any of our possible combinations as the successor to the throne or not. That's how I see it.
All of that does matter, though, unless the taint is cured in the Warden/Alistair--which is possible (Fiona), but at present does not seem likely. The longer one is tainted, the less fertile one becomes. If Anora is barren, too, and/or Alistair not particularly fertile even without the taint, well, that also makes it harder. Then the only way to have an heir becomes adoption, or hoping that there is a bastard somewhere.
However you or I or anyone else feels about the plot that the devs have planned, or the lore of the world, or the mechanics of the Taint, it doesn't matter. What matters is what the developers decide, and what kind of story they want to tell.
Remember that even though Dragon Age is a fantasy game, it's got a dark side. I won't say it's necessarily realistic, but some parts of it are, in order to have a grittier, harsher feel to the story. Lore dictates there isn't an heir, unless it's a bastard or adopted.