I'm a traditionalist, I enjoy the way Ryan and Krysty--and JB and Mildred--are portrayed already--loyal monogamous pairs. Sure, I am a fan of using sex scenes for a purpose--plot moving (like for when Ryan's seed is stolen from Krysty for breeding use) or for character use (like to show their relationship/interaction--like in Hell Road Warriors where they talk about concerns or there is humor--that whole 'breakfast' conversation was just plain PERFECT). And I definately prefer a skilled portrayal to just a graphic one. One of my favorite things in the series is where you see the pairs love for each other show outside of sex--like Ryan's reaction to Toulalan's kissing of Krysty's cheeks.
Given the DL's lack of decent medicine and the weirdness of all the mutational stuff we see in everything ELSE in the DL, keeping it monogamous would be the smartest 'survival' choice.
Besides, there is enough bizarre or wild sex already going on around them, that they are the 'stablizing' normal relationships in the DL.