Yeah you're right about sex being a method to create yang/positive energy....
Geumsun explained that “goblins die without yang energy” (ch 16) and use it to balance out their strong yin/negative energy. Since they can’t create it themselves, they either have to rely on reaper blood (which needs to be made into vitality capsules since it can be too strong), or sex with humans.
Hwan, on the other hand, is a "parasitic ghostly monster created from evil energy" who basically creates+then feeds off the negative emotions (not physical pleasure) of his victims.
The fear/terror Eunjae felt when he was raped, along with the painful emotions he experienced (and subsequent self neglect/punishment) after the death of his parents+Hoseung were caused by Hwan so that:
1. Eunjae’s soul-body bond would be weakened, making him easier to possess.
2. All the pain/negative emotions would make him “taste” even better when the time came to devour his soul.
No sex is an option for humans as well, but there’s other ways too: basically any experience that creates positive emotions=yang energy. That’s why, earlier in the story, San attempted to improve Eunjae’s mood by suggesting they go out for the day….“you can’t get yang energy from just sitting around at home!”

So ummmm....if sex is a way to create positive energy (sorry i dont know the word) then why was Hwan raping the MC in the begining?
Even if its rape, it should give sexual pleasure. And its all it needs for positive energy based on what i understand.