Ah yes! Let's take the one thing that the MC LIKES and is INSANELY TALENTED at and throw it out the window so he can't do it just so you can try and remove the awkwardness in your relationship, great job Kuro!!
Sakuya is a complete yandere, and it's not that she cursed him but won't let the divine messengers excorcise the curse, she wants him to solely be dependent on her so she can have him all to herself
Just a theory, but I think if he picks the one who is not originally from this universe, the universe will do a correction and kill the original one from this universe leaving only one, but if he picks the original, the one from the alternate universe will disappear back to her original universe.