I know exactly what you mean. I was also kind of hit in the face with his polyamory and Act 3 behaviour.
I guess Act 1 and 2 simply had so little Halsin-centric content that my brain filled in the blanks, just like you said, and I ended up imagining this mature and kind soul with a silly but domestic side... I don't know. I was hoping for an Alistair-esque relationship to be honest. Maybe that was my mistake, maybe it was the writer's... I think Minthara suffers a similar issue.