In 1685, Fairfax married Catherine Colepepper, daughter of Thomas Colepeper, 2nd Baron Colepeper, and they had seven children: Thomas Fairfax, Henry Colpepper Fairfax, Katherine Fairfax, Margaret Fairfax, Frances Fairfax, Mary Fairfax, Robert Fairfax.
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Born in 1635, Colepeper (often referred to by the alternate, Culpeper) was the son of Judith and John Colepeper.