Bill/Frank was hinted at as being a couple in the game (iirc), but I don’t think it was spelled out. Also (again, iirc), they were very minor characters (only referenced?) without any back story at all in the game. Going into the deep dive story of them is 95% show created. And I just think it is very out-of-left-field.
Joel gets a half episode back story. Ellie gets a 1 episode back story (1.1 episode if you count her birth). 2 rando characters who are essentially just mentioned in the game get their own bubble episode. Very much like Nikki and Paulo. The gay aspect, to me, just explains why they got their episode. It never would have happened if it were Bill and Francine.