(a) Of a Minor.–Whoever, in the special maritime and territorial jurisdiction of the United States or in a Federal prison, or in any prison, institution, or facility in which persons are held in custody by direction of or pursuant to a contract or agreement with the head of any Federal department or agency, knowingly engages in a sexual act with another person who–

(1) has attained the age of 12 years but has not attained the age of 16 years; and

(2) is at least four years younger than the person so engaging;

or attempts to do so, shall be fined under this title, imprisoned not more than 15 years, or both.

(b) Of a Ward.–Whoever, in the special maritime and territorial jurisdiction of the United States or in a Federal prison, or in any prison, institution, or facility in which persons are held in custody by direction of or pursuant to a contract or agreement with the head of any Federal department or agency, knowingly engages in a sexual act with another person who is–

(1) in official detention; and

(2) under the custodial, supervisory, or disciplinary authority of the person so engaging;

or attempts to do so, shall be fined under this title, imprisoned not more than 15 years, or both.

(c) Defenses.–(1) In a prosecution under subsection (a) of this section, it is a defense, which the defendant must establish by a preponderance of the evidence, that the defendant reasonably believed that the other person had attained the age of 16 years.

(2) In a prosecution under this section, it is a defense, which the defendant must establish by a preponderance of the evidence, that the persons engaging in the sexual act were at that time married to each other.

(d) State of Mind Proof Requirement.–In a prosecution under subsection (a) of this section, the Government need not prove that the defendant knew–

(1) the age of the other person engaging in the sexual act; or

(2) that the requisite age difference existed between the persons so engaging.