I think if Y wants X to come for an extended visit in 6 months, that Y should not date other people. If they live far apart, X needs to be able to plan this big move (end the lease on apartment/house, buy airplane tickets, ship belongs to Y's location, etc.). If I were X, I would find it very hard to plan to make a major, long distance move to be with someone who couldn't even promise exclusive dating.

I think it is very good X and Y are communicating on this. And that Y did not keep his/her dates secret from X. I guess if I were X, the question I would ask of Y, is "do you want me to come spend a year with you in 6 months time?" If the answer is no, well, end of story. It could be that Y's dating other people is a way of saying, "no, don't come!" If the answer is yes, move forward from there to negotiating dating.

Good luck.