100% of ALL marriages end in death or divorce.

But hey, people change over a thousand years!

Although that's not as farcical a statement as it may seem if you're talking about genuine immortality. Things may change when there's no longer a guaranteed end point to existence, but people are not locked in a fixed state, and so it would probably be extraordinarily rare and maybe a little weird for a couple to remain compatible and in love for century after century.

I imagine it'd be one of those cases like today, where a pair are sweethearts from toddlerhood on and die within hours of each other after turning 110.