Morrigan and Alister are basically an old married couple
No, no they are not.
Morrigan and Alistair are radically different people with very different upbringings and motivations whom by the nature of how they were raised were taught to view people like each other with suspiscion and disdain.
The idea that to be married requires mocking and hateful banter and the ‘I hate you why’d I marry you?’ Is a really gross trope long used by television to grant permission for two characters can be cruel and manipulative of the other. Any pairing based on this is at its core abusive.
From the minute she meets him, Morrigan is openly mocking of Alistair. She picks upon very sensitive subjects: his intelligence, his parentage, his love of the warden. I believe she does this for sevetal reasons: out of pure disdain, for this is the representation of the people who have been the reason for her to not leave the Wilds, why her and Flemeth moved all the time, and now shes forced to help him. And also out of insecurity. Morrigans greatest fear is in being posessed by Flemeth, is in no longer having autonomy, and despite how stupid they might seem to her men like Alistair can, and oft are far too willing to use their powers and attempt to break mages like her. Plus if youre playing a female Warden, she leaves the Wilds knowing that all of her plans hinge on having to have sex with a man who she already dislikes and made up her mind about.
She looks at Alistair in suspiscion, dislike, differences on a total moral basis, and perhaps a twinge of fear, and I’d argue just one of those traits makes a relationship a problematic one, let alone all of them.
Now on Alistair’s side, he has a built in minor suspiscion of mages, particularly apostates. He’s good with Wynne and Amell/Surana because theyre the kind of mages he was taught about, he knows what to expect and they are aware of his former templar status and what that could spell. But Morrigan acts in defiance, she eggs him on, insults and mocks him, and even though he has no intetions of turning her over to the Circle or templars, he is very open with the Warden about the extent of his distrust.
Alistair has spent his entire life having to experience people making him feel unworthy and uncomfortable. From Isolde, to the sisters in the Chantry, to the templars during training. He states that with Duncan and the wardens is the first place he felt at home and felt wanted, but even then, Duncan coddled him because of being Maric’s son. He shrugs off what Morrigan says and attempts to turn it back on her (unsuccessfully) as his own defense mechanism. His side of the relationship with Morrigan is one of imbalance, distrust and verbal abuse.
While we know that both Alistair and Morrigan prove to be loving and considerate people in a romantic relationship, thats because of the Warden getting to know them and showing them everything that they do not show each other: consideration, patience, respect, trust, an openness to their feelings and ideas. Morrigan and Alistair are not capable of doing this to each other, meaning there is never any good in their relationship to help them work through any of the bad.
So I see where your statement comes from Nonny, but the trope is a gross one and Morrigan/Alistair is an impossible and ultimately abusive pairing.