Ah yes, because some sagas had characters assuming atypical roles for their gender, it means viking society was genderfluid. Fuck off.
Sagas = stories about extraordinary people doing extraordinary things, sometimes based on fact but often mixed with myth. The old Norsemen also had a sense of humor so gender bending was used as comedy, much like it was done in Shakespeare's time or even in modern comedies. Like that tale where the giants want to wed Freya, but the Aesir dress up Thor as a woman instead so he can sneak in undetected and then slay all the giants at the great reveal. What a riot! Obviously played for laughs, as a good yarn, rather than as a statement of "The god Thor is genderfluid".
Trickery was a widely accepted strategy in Norse society. If you wanna get something or off someone but can't do it directly, disguises and other tricky methods were totally fair game.
There's also a big difference between "a thing existed" and "a thing was widely spread and accepted." Yes, men who practiced feminine types of magic existed, and they weren't instantly tossed in the bog for being fags, but they were generally seen as weirdos and your average person would have felt uneasy around them. However, if you tell a normal person in his face that he's a practitioner of feminine magic, he has the right to stab you right then and there because you gravely insulted him.
It is the same with ancient mediterranean cultures like Romans and Greeks. No, the Romans didn't crucify you for being a fag. But they'd fucking ridicule you because the one thing a Roman man should have is virtus, and virtus refers to manly virtues of strength, integrity, willpower etc. A fag doesn't have virtus, therefore he is a lesser man.
Of course, you were
allowed to be a lesser man. But that also meant you weren't able to assume societal roles normal men were allowed to (and expected to) perform. Wanna go through an ambitious senatorial career in Rome? If you're a fag, you won't stand a chance. Wanna be a badass viking leading a troop of men to foreign shores? Nobody is gonna join a weirdo who practices woman magic. People will have a sense of respect for a man who's good at that kind of magic, due to his power and occasional usefulness, but he won't ever be a normal part of society. People won't have the same kinds of relationships with him that they have with your average Ragnar from the neighborhood. If you choose a role like that, you will be an outsider.
People like these always like to pick singular examples from literature and history and then claim that this was some kind of norm. No, it wasn't.
Let's compare it with the ancient mediterranean again. Yes, there were some female rulers who led their own armies from the front. Actual badass warrior queens, both among barbarian cultures like the Celts, as well as Hellenistic cultures. But was it the norm? No. Just because a couple of queens went to battle doesn't mean there were scores of female soldiers in a Macedonian phalanx. Whenever you find a woman on an ancient battlefield, chances are that she's the
only woman on that battlefield. Or even the only woman in that entire fucking years-long war. Because it was not the norm, but the exception.
Now, what that means for an RPG is: the players should be able to assume such roles, because player characters are always exceptional people, not average people. Otherwise the players would roll a party of local peasants making daily field tilling skill checks, instead of brave adventurers descending into deep dungeons. But it also means that NPCs should react appropriately to such player characters. A player party of vikings where one character is a male seidr-wizard who likes to wear skirts, and another is a tough sword-wielding women with a scarred face? Yeah, these people are
weird. They're gonna be treated with curiosity at best, and with hostility at worst. Be prepared for the local kids to point at your characters and say "Look mommy! Look how weird these people are!", and the local tavernkeep hoping that you won't stay very long because he really likes having more normal people as his customers, they cause less trouble.