It's kind of a mess. And it's why I take issue with it as well. I didn't even really romance him, just did it for fun on good-durge planning to reload after. But I took the path where I didn't ask him about monogamy and romanced Astarion. I was pretty shocked because I genuinely didn't see any real lines about him being so 'casual' especially coupled with the lines you mentioned. (I also was not a fan of the Astarion trio at all given his story and how the writers punish players for perceived horniness with a romanced Astarion in every other scenario but when romancing Halsin.) Halsin as he is currently written is fairly toxic.