Would you date a vegan that though that its immoral to kill and eat animals.
What would you tell him/her?
I doubt it, because in my experience people so committed to such views expect you to not only respect their views but conform to them too.
It tends to be a one way street, my way or the highway, I've no problem with them eating what ever they like, but I will not put up with them not extending the same privilege to me.