why in the hell do they not show Agron and Nasir fucking?
Well it's not like we've seen much of the others. Seppia/Glaber were important to the plot, and Glaber/Illythia and Crixus/Naevia drew one of the most visually stunning parallelism between these characters.
I mean, maybe it's because I don't like them as much as you do, but I don't think there'd be any point in showing them having sex if not for the eye candy. They're happy and in love, it's more than you could say about any other character.