Headcanon: Sigyn: Past/Present/Future
(This is loosely based in the idea that MCU Frigga had foreseen her own death. And that Seidr users with a high enough mastery could see the future or potential futures.)
I believe that the reason Sigyn wasn't originally down for a marriage with Loki was because he saw his future. After living in a state of constant war and hyper vigilance. The first thing to clear the red haze from his eyes was Loki. And the shroud of pain and suffering that was his future tied tight around his neck like a noose.
As seers you can not try to change the fates of others or risk a compounding outcome. One way another the victorious one knew he would end up playing a minor role in the tragic story that was Loki Laufeyson. Try as he might he could not make himself not love him. Even as with every glance upon his face he saw more and more of his future. The inevitable conclusion of their little family and the love that grew besides the obvious and inevitable end.