@The topic:
That would depend on several factors. First, marriage is first and foremost a contract. Whether you marry for love or not, the difference between a relationship and a marriage is that marriage gives you legal rights and obligations, and official recognition by a community or authority. As we know, tolerance of same-sex relationships is generally not a problem in Thedas, but tolerance is different from official recognition. Such official recognition may depend on cultural traditions about what marriage actually "is for". If is is mainly about children and inheritance, a same-sex marriage would be unlikely to occur to anyone. I suspect it would not be a big matter in RL cultures either if not for the tradition of homophobia in the institutions that have historically recognized marriage.
Also note that in RL history, while the idea of marriage for love (alone) has always existed, it only became culturally dominant in the 19th century (and even then, in western cultures only). In earlier times, while love certainly was desired, it alone would not be regarded as a sufficient reason to marry. The dominant idea - reflected to this day in many countries' marriage laws - was that matrimony was a form of community of accrued gain. Inasfar as Thedas' cultures are inspired by Europe's renaissance and early modern periods (which I assume they are except in aspects explicitly written otherwise), I would expect Thedas to reflect that.
So, I expect that legally a same-sex marriage is possible in most Thedosian cultures with a tradition of marriage, because an "it's no problem" attitude towards same-sex relationships means it wouldn't occur to most people to forbid it, but you'd probably be looked at strangely, with the implied question "Why the heck would you even want that"? The nobility would probably discourage it strongly based on their need for continuation of the bloodlines, it would be regarded as inconvenient and undesirable at least for the heir and the spare. Again, if it occured to anyone in the first place,