asoiaf universe is so much tamer and softer than real life if you think about it. realistically, every noble house should have a ramsay or two due to the way they view smallfolk, and there should be a lot of rape happening in the nights watch, which is an all-male prison basically.
i disagree that asoiaf is "unrealistic" in the "opposite" way of the common and inaccurate "grimdark" critique from people who have allowed their memories of the show to completely overwrite their memories of the books. BUT, i also think "realism" is simply not the point of the series? it's not something that i see grrm trying to acheive and it's very strange to me how often it's brought up. westeros is not medeival england, no matter how deeply george draws from historical references for his inspiration. every castle is so big it genuinely boggles my mind when i read about it. every helm has 3ft of ornamentation. seasons last years and years before cycling, and their rates of change are unpredictable. there are ice zombies and fire wights and dragons and unicorns. i just don't see "realism" as a goal grrm has in mind. westeros is its own world, influenced by but separate from our own.
i could agree with you about wanting more sexual violence and predation in the night's watch but i don't think this is a weakness in the text nor a realism issue. i would just like to see jon snow passed around like a ragdoll. but i can tell you from a plot standpoint this doesn't happen because:
1) sending your chosen one protagonist to prison to get gang raped isn't going to tell the same story as sending your chosen one protagonist to be an indentured forest ranger at glacier bay national park
2) the night's watch has been reduced to an all-male prison but the original intention of the order was not as a gulag, but rather an honorable order sworn to protect the realm
3) george only thinks male gangrape is sexy when it happens to theon so that's who it happens to because i stay winning
and to your final point the thing is that every greathouse DOES have a ramsay!!!
rams is my special little boy but he is actually not noteable or unique in westeros, neither for his brutality nor for the protection offered him by his social status as a man, a legitimized bastard, and just a physically big dude (even if you don't subscribe to my leatherface faceclaim he was a miller before he got daddy's black card and spent his days doing hard physical labor, then was trained (poorly and informally) to weild a greatsword with full plate. he is physically strong and imposing in canon whether he looks like bubba sawyer to you or not).
roose's objection to ramsay's behavior is not that it's sick and cruel. his objection is that it's gauche and unsubtle.
is ramsay's kidnapping lady hornwood and marrying her at swordpoint meaningfully different than sansa being married off to tyrion while she's being kept as a prisoner of war? different than sixteen year old walda frey being sold to roose bolton in exchange for her weight in silver and a solidified political alliance? is it different than cersei being given to robert as a politically advantageous replacement for lyanna? is it different than rhaegar locking lyanna in a tower for the final months of her pregnancy? (ok the answer to the last question is yes it is different because the big plot twist wrt r+l=j is that rhaegar and lyanna were for real in love but he did ultimately betray her in the end by choosing the prophecy over her. but let's not get too off course!) is it different than lollys being sold off to bronn after the battle of the blackwater which grants him castle stokeworth in the crownlands for the low low price of the fat, traumatized, plain faced, pregnant, gang-raped "halfwit" daughter he need take to wife?
is ramsay locking jeyne naked in her bed chamber and raping her with his hunting dogs different from gregor clegane orchestrating the gang rape of a 13 year old innkeeper's daughter in front of her father? or tywin orchestrating tysha's gang rape and his proxy rape of tyrion? or roose seeing a random comely young miller's wife by a river and raping her under the still-warm corpse of her husband?
is anything rams does to his three brides meaningfully different from anything any lord in westeros does to his? sixty year old jon arryn accepting hoster tully's offer of his "ruined" second daughter and impregnating her like ten times before they finally had sweetrobin? gregor clegane's three unnamed dead wives? maegor's black brides? thirteen year old dany being sold by her brother to khal drogo in exchange for an army?
ramsay will forever be the worst thing that ever happened to donella, theon, jeyne, and the dozens of anonymous northern girls he raped and skinned with reek in the forests surrounding the dreadfort. to anyone and everyone else he is barely a political blip on the sprawling realm cleaving backdrop of the war of five kings. he is unique to house bolton only in the manner and presentation of his brutality. and even that isn't truly unique to him!! i would say that annointed westerlands knight gregor clegane is not any more or less refined and noble mannered than rams, and the things he does to women, children, and smallfolk are just as horrifying as rams' house of 1,000 corpses hobby. disgraced maester qyburn and his herbert west/dr. satan schtick is another great example, actually, of a very open secret being protected by wealthy, powerful nobility.
ramsay, like craster, serves not to show the unique and unprecedented cruelty of one character, but to expose the general casual normalized cruelty of westeros as a whole.