She's not Thor's true love in the myths either. He got around.
Viking men, as the scholars so charmingly put it, had access to other women outside their marriages, and it wasn't really considered adultery. Naturally, the same standard did not apply to Viking women.
Marriages in the Viking Age, like in many societies, was about securing larger family ties. I think they wanted to ensure they could live with each other when arranging marriages, but love wasn't really a consideration (though it undoubtedly happened among some matches).
As far as I know, there is not a tale among the myths in which Tor and Sif are not married (and, for example, dating, as it were).
Thor doesn't 'transition over time'. Jane gets ditched by Odin and Thor gets sent off with Sif in the very same issue. Keith Kincaid was never a tasteful send off, he was a plot device to section Jane off and away from Thor so Thor couldn't be with her even after they got their memories back.
I hadn't realized it was in the very same issue. But yes, as I understand, Kincaid is not so much a love interest as a McGuffin. And if you think about it, that's not really fair to him either.
But as I said earlier, it is possible for them to write it so that it unfolds more naturally, and in a more dignified way for everyone involved. (not that I want this to be the outcome, just that it's quite possible to do so)
Fraction for all his faults gets the dynamics right. The point of that relationship is to show us that Thor uses Sif as an emotional punching bag. There is no equality in character there. Sif is desperate for Thor and while Thor loves her greatly it isn't the same kind of affection. Fraction has Thor call Enchantress with the same endearment as Sif, 'woman'. Compare that to Thor's only scene with Jane under Fraction where he takes time to explain things to her and says 'Ah, good Jane'. Jane is a character who just won't accept being called 'woman', she will call him on it. 60's Jane was the same.
Well, I haven't read these other stories in Fraction's run (though I think I've seen the panel you describe regarding Thor and Jane), so it's hard for me to evaluate. Within this one July, 2011 issue, though, I do not get the sense that he is using Sif as an emotional punching bag. Most of their dialogue concerns this pathetic army that they're trying to train. Regarding emotional content, for Sif's part, she tells Thor that Loki, who is a child here, wants very badly to impress him. And then she expresses concern (matched by the artwork of her face) about a wound in his abdomen that he's clutching.
So I could see that being consistent with Sif being more emotionally invested in the relationship than Thor (though there could easily be other interpretations, too). But it doesn't look like abuse, at least from this one issue.
I don't get a strong sense that Thor loves her from this one issue, though I have seen isolated panels that are also clearly Coipel's art where he does state that. So I continue to not really understand why this is such a great, true love.
Regarding being called "woman" vs. calling Thor on it, I seriously doubt movie-Sif would put up with an insult. I have seen many panels of Jane not taking any nonsense from either villain or Hero, so I agree with your assessment there. (So between these two ladies, Our Hero might be in trouble!
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Thor/Sif works in the childhood sense because they are both hotheaded and arrogant. When Thor goes through his change and becomes a better person that perspective changes.
That may well be, though I will also say that cuts both ways. This is from an interview Kevin Feige did with Collider:
"Collider: How has Thor’s growth into a man shaping what you wanna do in the sequel, in terms of him being an adult?
"Feige: Well it's sort of the crux of the whole sequel. . . .it's Thor and Jane, to continue that dynamic. . . ."
That could be interpreted as asserting that Jane is Thor's love at the end of his adolescence, and in later movies, when he is an adult, maybe he feels differently.
However, you make a good point in that events of their childhood are on the order of a millennium ago. People change and grow apart. So maybe after I have a chance to read it I should also look around to see how their relationship is being portrayed now-a-days.
As I said, I don't really see how Sif is Thor's true love, especially from the material in the movie. But I'm willing to try to understand where people are coming from.