Green’s Dictionary of Slang

why get married when sexually permissive women are so available?

[1680		Bunyan Life and Death of Mr Badman (1696) 208: ‘Who would keep a cow of their own that can have a quart of milk for a penny?’ Meaning, who would be at the charge to have a wife that can have a whore when he listeth? [F&H]].
1938		J.H. O’Hara BUtterfield 8 sic capped up BUtterfield178: It sounded better than the American: why keep a cow when milk is so cheap?
1957		(con. 1943) A. Myrer Big War 87: ‘I’m a compromised demoiselle. Canned, as the saying goes. Unless you want to marry me. Want to marry me?’ ‘Sure. Why keep milk.’.
1959		J.H. O’Hara From the Terrace 485: ‘Married and all that?’ ’No, still batching it. You know the old saying, why keep a cow when milk is so cheap?’.
1991		D. Kirby Jottings 24: People would often ask him why he wasn’t married, and he would make the reply, ‘Why keep a cow when milk is so cheap?’.