I think the issue was that his blood would turn golden the longer he repressed his lust, so in reality as long as his blood remains that color he IS kinda in danger. That's why MC was trying to sleep with him I think.
Wait now I have a new theory, I think sleeping with Tei is enough to turn his blood back to red because if not, how are his surviving brothers blood not turning gold then and only his? All of them have the eyes. If the other two brothers are just sleeping with their significant others without the necessity of reproducing then I think sleeping with Tei can actually cure it. But the real question is will Tei be able to neutralize the toxin before Shuraka can fully be cured or it will poison him first before he can cure Shuraka? Remember all three brothers agreed to only have Banadal as the next Sun meaning, neither all three will have another children aside from Banadal.
Wait is shuraka dying what is with this blood poison?, I knew he had golden blood cuz him surpassing his sexual desire with that smoke thing but what does tei mean by poison??