In the context of church activity and membership, I'm NOT saying one spouse has to or even should leave the LDS Church if the other spouse leaves or isn't a member. I'm not saying that at all. For some, that might be the only option - but I don't think it is even close to the "ideal" solution for the vast majority of people, and I would not recommend it for the vast majority of people who are in that type of situation. I believe people need to be as true to their own consciences as is possible, while being willing to make acceptable sacrifices to stay together. (I'm not including abusive situations in any way when I say that - which is a totally different discussion.)
So, my advice to all who are or will be married is:
Cleave unto her / him, regardless of religious differences.
- or, as a friend whose wife left the Church said years ago, work to make your marriage stronger despite your differences in whatever way you can.
Work to become truly "one" - since I believe God won't split two-become-one. Our temple work (for the living, but especially for the dead) is the tangible "proof" of our belief in that principle. I believe it applies just as much to any two living people as to any other two dead people. Become "sealed" in practical terms, and I believe you will be "sealed" by divine declaration in the end.