A minor under the age of fourteen/sixteen is incapable of giving consent/agreement to engaging in sexual conduct which is otherwise prohibited by law and the agreement/consent of such minor to such activity should be disregarded by a jury in determining the question of the defendant's guilt.
This instruction is appropriate for rape and other sex crimes when the victim is a child below a particular age. This age is fourteen for prosecutions for rape in the first degree and sixteen for prosecutions for rape in the second degree and other sex crimes. See 21 O.S. 1991 & Supp. 1995, §§ 1111-1114.
The Oklahoma Court of Criminal Appeals held in Kimbro v. State, 857 P.2d 798, 799 (Okl. Cr. 1990), that as a matter of law a child under the age of sixteen cannot consent to oral or anal sodomy. In doing so, it overruled Slaughterback v. State, 594 P.2d 780 (Okl. Cr. 1979).
As to proof, that's a matter for the jury however, in the above instructions to Oklahoma juries for sex crimes published by the Judicial Review Committee, it's clear that any proof must be of a sex act, not just fantasy.