if you want a serious answer (spoilers):

You can only have sex with him if you help him out in the war (bull village destroyed) or if you get the good ending of the war (peace between both tribes) and have good reputation with the lizards. You'll know you unlocked the sex scene when Nauxus asks you to go to his throne room at the very end of the war (after you forgive both tribe leaders for being impulsive and starting the war.)