Warn them!

"It is too late, when the crisis comes, to begin telling a wife or husband or mother or friend, that your love all along had a secret reservation - "under God" or "so far as a higher Love permits". They ought to have been warned; not, to be sure, explicitly, but by the implication of a thousand talks, by the principle revealed in a hundred decisions upon small matters. Indeed, a real disagreement on this issue should make itself felt early enough to prevent a marriage or a Friendship from existing at all. The best love of either sort is not blind." The Four Loves, C.S. Lewis