Did a random bit of legal research following on from the whole Todd Akin brouhaha.
Just for historical interest, this is from Scott J in US v Dickinson, an 1820 judgment in the Superior Court of the Arkansas Territory:
The old notion that if the woman conceived, it could not be a rape, because she must in such case have consented, is quite exploded. 1 Hale, P. C. 631; 1 Hawk. P. C. c. 41, § 8; 1 East, P. C. p. 445, c. 10, § 7; 1 Russ. Crimes, p. 677. Impregnation, it is well known, does not depend on the consciousness or volition of the female. If the uterine organs be in a condition favorable to impregnation, this may take place as readily as if the intercourse was voluntary. Med. Jur.
The references are to Hale's Pleas of the Crown (1736), Hawkins' Pleas of the Crown (1716), and East's Pleas of the Crown (1803) and to Russell's Treatise on Crimes and Misdemeanors (1819).
East puts it pretty bluntly:
It was formerly supposed that if a womman conceived it was no rape, because that shewed her consent; but it is now admitted on all hands that such an opinion has no sort of foundation either in reason or law.