Ask your son what the current definition is per UCMJ. I wonder if it has changed since I was briefed on the topic (eons ago).
UCMJ
925. ARTICLE 125. SODOMY
10. Punitive Articles
(a) Any person subject to this chapter who engages in unnatural carnal copulation with another person of the same or opposite sex or with an animal is guilty of sodomy. Penetration , however slight, is sufficient to complete the offense.
(b) Any person found guilty of sodomy shall be punished as a court-martial may direct.
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And still on the books, I think!