Not true. The FBI's definition is:

"The penetration, no matter how slight, of the vagina or anus with any body part or object, or oral penetration by a sex organ of another person, without the consent of the victim." [1]

1. https://www.justice.gov/opa/blog/updated-definition-rape

So the FBI does not consider female and male perpetrators who engage in non-consensual sex with a male victim only using the victims natural external genitalia?

This seems pretty stupid.

Can anyone corroborate this claim? As in do they not or actively refuse to prosecute these incidents? Or are they "sexual assault" but not "rape"?

Wouldn't the fact that this is a direct quote from someone high up in the FBI, and is being hosted on the website for the U.S. Department of Justice be enough corroboration?

I think you're reading it wrong.

The penetration [..] of the vagina or anus [..] without the consent of the victim.

Nowhere does it say that the body part must belong to the victim.

FBI's definition does not apply in England.

True, I missed the "English law" bit.

This means the definition of rape depends on the country you are in. For the UK I think it's really arguing semantics though, because it is defined as "assault by penetration" and has the same penalties as rape. [1]

1. http://www.legislation.gov.uk/ukpga/2003/42/part/1/crosshead...