He says later that was when they were both kids and now he feels this is wrong
Same here. I thought the only explanation for that double standard here could be that, while it was ok for a "yang" man to get together with a "yin" man (because they were born to be mutually "compatible"), it was not okay for a "yang" to force an "ordinary" man (someone who was neither "yang" nor "ying") into a same sex relationship, let alone of the ordinary person was his brother (as in, that family relations are only overridden when both of them are "not ordinary".
As in, if it was an omegaverse, it would be as if a mega alpha guy forced a beta guy into a relationship, and moreover that beta guy was his own brother
I have a question, when they were both kids, the bottom said that he wanted Narye to become his Yin, didnt he? So obviously he was totally okay to be with his brother. But when their role switched, now that he was the Yin, they being brothers suddenly become taboo. So, he was effectively grooming his younger brother to be his Yin, right? Not so innocent, wasnt he?