Greg and Marsha (or is it Marcia?)
So Greg was hoping Roy was really Marcia's father so then they could "get together".
But even if he wasn't (which he wasn't), Greg and Marcia still had different mothers and different fathers so they were not related anyway. Were they? They could have done whatever they wanted to. It's more of a psychological thing I think. Either way.
And why did the Brady girls change their last names to Brady (from whatever) anyway?