My argument against allowing for gay marriage of Alistair to be ported is a bit... harsh, maybe, but it's not out of hatred for gay couples. No one is allowed to marry Alistair except for Queen Anora, or the Female Noble Warden. Ostensibly, these are people he could have acceptable heirs with. Now, he might be barren, and the female Warden definitely is, and Anora might be too. But, the nobility of Ferelden don't know that.
If Alistair in the base game romances anyone other than human noble woman, he will break up with them if not "hardened." And he won't marry anyone but human noble woman or Queen Anora, and will only marry the human noble woman if he is hardened.
If Alistair were not in a position where producing heirs/breeding were a priority, if he weren't potentially the king of Ferelden, I'd be all for saying "sure let's port this into the game." But here's the thing: An acceptable heir can't come from an elf, dwarf, or mage. It can't come from a commoner. And men can't have babies unless they are transexuals. Only a woman can bear an heir. Adoption might be a possibility, but I don't know how Thedas in general or Ferelden specifically handles that. I do know that in real-world historic times, adoption was of adults and used to secure an heir. But the problem here is that Alistair is the last of the Theirin line, so he doesn't have a convenient cousin or the like he can adopt.
And if you've read The Masked Empire...
Now that said, is it unfair? Yes. But nobility are expected to breed, because as long as they inherit leadership, having an heir means the government will be stable, and people won't have to worry so much about civil war erupting. And I do hope that commoners, at least, are allowed to have s/s marriage. The trouble is that nobility, at least (no idea how commoners are handled) are allowed to have a s/s lover on the side, but must make a straight marriage and "take one for the team," even if they are gay/lesbian and not at all interested in the opposite sex.
So, my reasons aren't that Bioware couldn't/shouldn't change content due to the difficulty (though that's certainly a consideration, and things have changed a lot since even just 2009 in terms of what is acceptable and what isn't), but that lore-wise, and story-wise, at present (disregarding any lore changes that may be made for the fourth Dragon Age title, or beyond--which it would be nice if they did, to allow for s/s noble couples who adopted an heir, etc.) the two kings thing wouldn't work. It's not fair, but Bioware has pretty much made it clear that it wouldn't happen.
You'd have to have the support of the other nobles for any marriage to occur, and you wouldn't, because they would want there to be an heir. You might be able to best each individual noble and his/her champions in combat, but you're not going to do much good against the armies they raise up against your rule when you threaten the stability of the nation with the lack of an heir (thereby making it so that should anything happen to one or both monarchs, the throne is up for grabs, and civil war erupts).
I hope that it's clear that this is not a hate-fueled rant. I do believe that BioWare could have considered the adoption concept (again, historically adoption first appeared as adults, not minors, being brought into a family without heirs, to be the heir to that line), thereby allowing even nobles to have s/s marriage. But, as they have written things, from what I've read in Masked Empire, I am not even sure how s/s relationships are handled in terms of commoners--hopefully they are allowed to at least live together in peace, and preferably allowed to marry.
But definitely don't stop fighting the good fight out there, because you might inspire the "rules" of other nations than Orlais and Thedas to be different, when we visit them in future games. So, then there could be same sex marriage, with adopted heirs. Or same sex nobles could be required to marry a third, opposite-sex person for breeding purposes (it's horrible, regardless how you look at it, but nobility basically are treated as breeding stock even in the modern world... what was the first thing people wanted Will and Kate to do? That's right... have a baby...).