Yeah also in norse mythology Loki and Sif had an affair. The evidence is the Lokesanna quote, Poetic Edda, stanza 54. In this, Loki basically accuses Sif of having an affair with him. Loki to Sif -
"I alone know, as I think I do know, your love besides Thor, and that was the wicked Loki"
In the poem, Sif makes no comment on this at all, not even to deny it. Also, earlier in the poem Odin (in disguise) tells Thor that Sif (Thor's wife in the myths) has a lover at home. Also, the whole Loki cutting Sif's hair has a few hints in it too, in norse myth, cutting off one's hair was seen as a sign of adultery then and that it's questioned just how Loki was able to get so close to Sif that he cut off all her hair.
So, basically, I think there is some quite strong hints in the myths themselves that Loki and Sif had a relationship and going off the Lokesanna quotes, that it was more than just a one night stand.
Finally, Sif has a son called Ullr who is the stepson of Thor. The father is unknown, but he is most likely not an Aesir (the race of Asgard). According to Myths of the Norsemen, by Helene A. Guerber (orig. published in 1909), Ullr loved the cold and his father is a Frost Giant. Also, Ullr's latinised name, Ollerus, "is described as a cunning wizard with magical means of transportation". So a few hints here of who the daddy could be... It could be Loki or it could not be.