For the most part, Thor wasn't wrong. For all those reasons and more, Sigyn was perfect. Except for her very faulted taste in men, apparently. But, even Loki couldn't control the rather noticeable twitch at Thor's suggestion that if Loki didn't jump on it, he was going to see if Frigg would set her up with on of his sons. As much as Loki was trying to push Sigyn to go to someone else, he really didn't want to actually think about that. Dammit.
And there was no good way to answer Thor's question either. There was no way to look at him and say that the last time he genuinely wanted someone and had a good shot at keeping them, some stupid high-and-mighty twat got involved and that woman ended up marrying someone else instead, rather suddenly. Instead, he offered, “because she has terrible taste in men?”
Loki figured that Thor might argue that, so he tried to pretend that was a joke, “seriously, though. What exactly do I bring to the table? I'm not reliable, I'm not in one place for very long, I certainly don't do monogamy well,” Loki sighed and ran his hand through his hair, “I already have two disastrous failed marriages under my belt, Thor, and I have not raised a single one of my children to adulthood before they were removed from me in some capacity.” He shook his head.
“That's not exactly ideal spouse material. I would hurt her... I already have by trying not to.” Then, he sighed, “and then you forget... you sister is a whole other fucking obstacle of her own.”