Originally Posted by Moonwing
If we think in the range of theory that Sif=Jörð we do not know if he was the only one who she have an affair with, its probably just the most scandalous one considering sides involved. Earth incarnation in loveless, mandatory marriage neglected by her husband/son (it is a miracle that they had Þrúðr together) it is easy to see why she fall in affair with Loki, and probably many others.
As for why Thor cared more that Sif's gold hair was gone than him finding out that she had a lover, well it`s not his business with whom his mother will chose to sleep. But hair-its her pride and beauty and she was devastated so it was his duty to avenge it=).
By the way, I am stile interested in why marvel do not use Járnsaxa at all. It can be so interesting cross culture pairing without need of human part. And it will free Sif as even in comic her relationship with Thor is strange on-off thing.
Marvel HAS uses Magni and Modi. Magni twice for that matter. But it was never clear the first appearance of the Sons of Thor were Sif and Thor's or not.
The Magni only was the son of Amora and Thor, that was in the future with Thor having knowledge of his future son