There's no lore, no anything supporting that marriage serves as primarily a political function in Thedas. There are several unpolitically motivated husbands and wives in the game. There's no more backing your claim than anything else. Nor is there any evidence there is no taboo against extramarital affairs. You're making up a bunch of crap about the setting that isn't even there, and there is no evidence of. Find me something that backs up your claims about Thedas.
Hmm, let's start with:
Loghain and Rowan have an affair in The Stolen Throne
Maric has an affair in The Stolen Throne
The PC can select from a number of affairs in Origins, and can even become the OFFICIAL "something on the side" of the King of Ferelden. Zevran will actually recount some of his affairs to you if you ask him. Leliana indicates having had some as part of being a Bard.
Wynne will mention (fondly) one of her affairs, which you find out in Asunder was with a templar--it resulted in a child
Branka goes wandering in the Deep Roads with her lover
From the wiki: The dwarves of Orzammar are noted for their polyamory (the keeping of multiple intimate partners), especially among the upper classes. While a dwarf has only one legal spouse, many nobles keep concubines, typically noble hunters, who are raised up to be part of the household and have their names recorded in the Memories. A dwarf's caste is determined by that of their same-sex parent, so many casteless dwarves offer themselves to nobles or to other higher caste dwarves as sexual partners in the hopes of producing a child of the noble's gender. As dwarven fertility is in dangerous decline due to chronic exposure to darkspawn corruption[4] as well as because of constant war with them, any noble child is considered a blessing, and such offers are rarely turned down."
Oghren can have an affair with Felsi
Maric has ANOTHER affair in The Calling
DA2 contains a large number of characters who have or can have affairs of varying types: the PC, Ninette de Ghyslain, Isabella, Fenris, Anders, Thrask, Merrill, your MOM (who also has very little negative to say about you having affairs with whoever--even with an elf), Gamlen (who hangs out at the brothel quite a lot), Aveline (although they do get married)
Empress Celene has an affair (with an elf) in The Masked Empire that results in a lot of political hoorah--not because it was an affair, but because of the whole "elf" thing--and even then it was a political move to discredit her by a specific antagonist, NOT a general reaction
The entire series is full of this kind of stuff, and NOBODY ever starts gasping in horror over it. No Chantry priests preach against it. Nobody's much concerned with it (apart from when it's opportune to use in a political maneuver). The devs have even mentioned that, say, in Tevinter it's generally accepted to have all kinds of relationships provided you're discreet about them.
So, yeah, there's a TON of evidence that obsession with sexual exclusivity is not a cultural norm in Thedas.