I really don't see Dirk's arc being about his sexuality at all. Dave's arc, for example, is largely about trying to move past the ideals that were forced on him growing up, and accepting his attraction to men is a big part of this. In the epilogues, Roxy doesn't even get much focus but it was made very obvious that his gender was the main focus of that character there. Dirk's gayness, in comparison, just seems like a simply fact of the character. Dirk doesn't have to discover it, or come to terms with it, he starts the story already in that position. The only time Dirk's sexuality really factors into character arcs is in Dave asking him to help come out (which is obviously more about Dave) and in Roxy's former lack of respect towards his sexuality. The situation with Roxy is the only place I really see it factoring into his character, with him being unwilling to call him out on it, but I think that reflects more on his own negative view of himself than of a negative view of his sexuality. He doesn't blame Roxy or call him out on it because Dirk sees himself as the one in control, as a manipulator who is constantly at risk of harming his friends, which blinds him to their possible faults. The only reason I think Dirk could be seen as "the gay one" is because being gay ISN'T really a big part of his arc, he is just openly gay the entire time we know him (unlike other queer characters who have to figure it out to some extent).