All of it, the entire speech, was predicated on Frigg's own personal dislike for Loki. While there was a bit of truth to a few things, she took them to an extreme. The idea that Loki would simply leave Sigyn to fend for herself, for example. Thor knew, as Frigg did, that his friend wasn't much of a fighter. But he also knew, as Frigg did not, that the reason Sigyn had to go berserker was because Loki had been hurt attempting to rescue her. And that was before they got all squishy-feeling about each other, when Sigyn was a girl he just bickered with. Loki had depths that Frigg didn't know about.
Actually, Loki had depths that Thor was pretty sure Loki didn't know about either.
He sighed. "No, I wouldn't let Thrud be with 'im, but that's got nothing to do with 'is history, and everything to do with 'er personality. My baby needs somebody..." Thor trailed off, then shrugged and finished lamely, "special." He knew what his daughter would need, he just didn't know how to put it into words.
"But 'e's not interested in Thrud," Thor pointed out. "'E wants Sigyn. And she wants 'im. And really, I'm curious just how you think you're gonna manage to keep 'em apart permanently. 'Cause I don't know if you ever noticed this? But when you tell people they can't do something? It makes 'em want to do it even more. I got kids. I know. That's exactly what happens. You didn't put a stop to anything, sister. You made it forbidden fruit."
Leaning back on the seat, he stretched out his legs and crossed them at the ankle. "'E didn't have 'er in his bed before, but I bet 'e will now. Within days, prob'ly."