He was in the process of committing sexual assault.
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Seems the hooker was doing what he was paid to do.
Not sure how that constitutes sexual assault.
Caveat Emptor.
He was not doing what he was paid to do if he was presenting himself as a woman. It was deception. Clearly the marine would not have participated if he knew the prostitute was a man. That should be considered sexual assault. It was not consensual. The marine did not consent to having sex with a man or even a transgender nutcase. He was tricked into it.