shounen-ai or yaoi manga with a nearly time travel: I'ts about an isolated island where the inhabitants life like in the edo-period. The inhabitants doesn't know that the world is technological advanced.

An man came to the island for work or research or something and someone there fell in love with him. In the end he find out that the world is bigger than his small island and he cannot return there (at least permanent).

Sakoku Renai?

Yes, this is it, thank you; because nothing of the story is said in the description i wouldn't find it again.

Thank you thousand times dione!