I really liked cariad's idea of a Latin word. So, following that theme, I found these that I thought you may consider:

debeo - to be morally bound to
spondeo - to betroth, promise, pledge
perennis - everlasting, uninterrupted
sempiterna - eternal
verus - true, real, proper, right
perfectus - complete, done, without flaw
iunctus - connected, joined together
conjugium - wedlock
licet - it is right, just
totus - whole, entire, complete, all
plene - completely, wholly, fully

Hope this helps you out!

So will you share a pic when after you have them engraved?