In sod's wiki link:
The views which contributed to rape laws not being applicable in marriage can be traced, at least partially, to 17th century English common law, which was exported to the British American colonies. The 17th-century English jurist, Sir Matthew Hale, stated the position of the common law in The History of the Pleas of the Crown (1736) that a husband cannot be guilty of the rape of his wife because the wife "hath given up herself in this kind to her husband, which she cannot retract". The principle, no record of which is found earlier than Hale's view, would continue to be accepted as a statement of the law in England and Wales until it was overturned by the House of Lords in the case of R. v. R in 1991,[1] where it was described as an anachronistic and offensive legal fiction.
The strong influence of conservative Christianity in the US may have also played a role: the Bible at 1 Corinthians 7:3-5 explains that one has a "conjugal duty" to have sexual relations with one's spouse (in sharp opposition to sex outside marriage which is considered a sin) and states that "The wife does not have authority over her own body, but the husband does. And likewise the husband does not have authority over his own body, but the wife does. Do not deprive one another (...)"[2] - and this is interpreted by some conservative religious figures as rejecting the possibility of marital rape.[3]
That's the religious argument they fall back on, in claiming there's no such thing as marital rape. Oh, the ways they apply those bible verses so conveniently!
