As it has been said by others here it is highly unlikely that same sex marriage is a thing in Theadas, while it’s true that it’s never been addressed directly in lore what we have been told is that same sex relationship are viewed a quirk, in essence it’s something that is tolerated to an extent but it is given no "official" backing religious or otherwise.
You could look towards the attitude of the ancient Greek’s towards homosexuality as an example of what is probably the situation is in Theadas, it was tolerated as long as you did your duty and got a proper wife and produced heirs.
Farther more you need to remember that the point of marriage in Theadas, at least for the nobility, is a matter of creating alliances and the production of children which a same sex marriage would fail to accomplish.
Edit: Slightly ninja'd by katerinafm... 