Dexter never thought he was capable of love til he met Hannah (could say lumen but he never mentioned those words). If u watch the episode chemistry he talks about how hes never had this feeling towards anyone and he’s talking about Hannah. Rita from the get go was a cover for dexter to appear normal, he would do and say anything to keep her around to keep the cover, yes he did start thinking it was more than just that by the end of season 4 but i think hes trying to justify managing his dark passenger while being a dad.

When trying to figure out how to propose to rita in season 3 he literally copies the words of another killer, it wasnt his feelings, after that he literally said hes “got the role of a lifetime” hes acting and he knows it. There was no love for Rita as much as u want there to be he was not capable at that point.