[/quote] this whole poly/sex driven side of him is out of character. It doesn't really fit with his personality.[/quote]

This.

I think we can have an additional quest where Tav and Halsin can have a deep conversation regarding their relationship near the end of Act 3 before the ending and help him overcome his fear of commitment.

In this way people who want to have a poly relationship with him can just avoid the quest and still have that poly relationship with him.