OK, I see from Google the FBI changed it in 2013. Not very "long" but whatever. The actually changed it from the definition involving "carnal knowledge". Carnal knowledge requires vaginal penetration, that is what the legal term means.
carnal knowledge
n. from Latin carnalis for "fleshly:" sexual intercourse between a male and female in which there is at least some slight penetration of the woman's vagina by the man's penis.
http://legal-dictionary.thefreedictionary.com/Carnal+knowledge
Last year, the federal agency’s Uniform Crime Reporting program started collecting rape data under the new definition. The new definition reflects an expanded interpretation of what types of penetration constitute rape and includes male and female victims.
The old definition of rape included only female victims and was narrowly defined as “The carnal knowledge of a female forcibly and against her will.”
The new definition is broader and is defined as, “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.”
http://www.adn.com/article/20141112/new-fbi-definition-increases-alaskas-already-high-rape-rate
That is has changed only underlines that there is NOT one, immutable legal definition. Note also that under this new definition although now a man can be raped it still requires penetration, so if you accept this new definition as the one and only "right" one you are saying that a man can only be raped by another man, or a woman using a strap on. A woman forcing a man to penetrate her, or go down on her, is not rape. Is that your position?
There simply ISN'T a single definition of the word, legal or otherwise. In fact most states don't even have a crime of rape on the books at all, the more common legal term is actually "sexual assault". You can't technically be convicted of "rape" in Washington DC, Texas or most other states, only "sexual assault". This doesn't mean they don't have "rapists"; in most states the words are actually legally synonymous.
And in many places that do still have a crime of "rape" on the books, women cannot technically rape men. This is the case in England, for example; they have an equivalent offence for women with exactly the same penalties but it is NOT technically, legally, called rape. This doesn't mean that men cannot be raped in the common use of the term, the common use of terms is often very different to the legal one.
The legal definition of the word is far from fixed, it differs in every jurisdiction, and the common use is even more varied.
The 1 in 5 study refers to "rape" AND "attempted rape" so I would have thought it was clear what it was concerned with but if not, it did indeed cover unwanted sexual assault that stopped short of the archaic legal definition of "rape".