Origin unknown. Attributed to Sydney Smith in Speaker's Handbook of Epigrams and Witticisms (1955) by Herbert Prochnow, p. 190. Variant reported in Why Are You Single? (1949) by Hilda Holland, p. 49: «When asked by a young man whether to marry, Socrates is said to have replied: "By all means, marry. If you will get for yourself a good wife, you will be happy forever after; and if by chance you will get a common scold like my Xanthippe—why then you will become a philosopher."»
Misattributed
Variant: By all means, marry. If you get a good wife, you'll become happy; if you get a bad one, you'll become a philosopher.