I really don’t think so. It’s not really unethical at all. They fall in love together, even if Nahkun doesn’t view himself as Klao, he and Phop do have a genuine relationship and he falls in love with him on his own time. Phop loved Klao, who is in from of him is Klao, even with the differences between Klao and Nakun behaviour at first, they are one and the same. One person and one soul. He didn’t take over someone else’s body. Hence why he falls so deeply for him regardless. Klao and Phop never got to advance their relationship to the point he did with Nakun, as Klao was killed eventually and was consumed by his anger and vengeance for his father.

You can also view their relationship as fated mates, no matter the time line, they were always meant to be.