I'd just hold her close and thank her for demonstrating such trust in me and accept it. If the relationship still exists I'd only worry if it interferes between you and your wife. If not, I'd just take things as they come. It's simply another facet of the woman you hold dear. Let it be.

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As to marriage or celibacy, let a man take which course he will. He will be sure to repent - Socrates

Love is not looking in each other's eyes, but looking together in the same direction - Antoine De Saint-Exupery