That's true as well, but Alistair was like that with Morrigan from the very first time we encounter her in the Korcari Wilds looking for the GW treaties. When I think about it, Alistair has no more reason to be anymore suspicious in the juicy gossip dialogue, than he was at the first Morrigan encounter, there's not much else he knows about her, especially considering Flemeth and Morrigan saved him and the GW. Only when Morrigan asks the GW to kill her mother and especially at the DR, asks her/him to get Alistair to perform the DR with her, should there arise more of a suspicion. In the JG dialogue, the only reply Alistair actually likes from the GW is when s/he says, "I don't have to like her, she's useful".
There's that. Alistair has been suspicious of Morrigan since the beginning, in part because of her being an apostate/Witch of the Wilds:
Alistair: Morrigan... do you trust her? Think about it... maybe Flemeth sent her with us for some other reason than she said?
Warden: You're probably right.
Alistair: And you're just letting her follow us? An apostate and Maker-knows-what else?
Warden: Your Templar training is showing.
Alistair: It is not! Ok... maybe you're right.
But he has a growing dislike of Morrigan from her general taunting of him that also colors his perception of her. So, yeah, he's extremely biased but happens to be correct, at least in that.
What's really interesting when talking about suspicion, in the dialogues of the GW with Morrigan and Alistair, Alistair is a bit more reticent when trying to get him to be a bit more expository about himself than Morrigan is, yet he holds her in suspicion. Unless you really pry into him with second and third questions. does he cave in to giving answers. This does not however make him evil, just personal to himself.
Yeah, Alistair keeps his secrets too, though his timing in revealing them is better (well, only if you bring him to Redcliffe
). He did tend to deflect a lot on his past. When asked, he says he kept his birthright a secret because he wanted the Warden to like him as Alistair, not the bastard prince. Morrigan keeps her secrets for different reasons. Interestingly, when doing the Dark Ritual with Alistair, she'll be surprised when Alistair brings up the child, knowing very well it would make him less likely to agree:
Warden: Alistair has agreed to your...request.
Alistair: Wait. I want ask about this child. The one you want.
Morrigan: Interesting. Honesty wouldn't have been my first choice