The reason why Marco Rubio is a citizen, not a natural born Citizen, is that his parents were not U.S. citizesn when he was born here in the U.S.
Three types of statutory citizenship are recognized by our government: native born; naturalized; and citizen-by-statute. All have equal rights. All can serve in Congress, either as a Representative in the House, or as a Senator in the Senate.
The following link will take you to the government’s own Immigration Service web page describing the three types of citizenship.
http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/?vgnextoid=a2ec6811264a3210VgnVCM100000b92ca60aRCRD&vgnextchannel=a2ec6811264a3210VgnVCM100000b92ca60aRCRD
Natural born Citizen is NOT a type of statutory citizenship. Natural born Citizen is ONLY an eligibility requirement for the U.S. Presidency per Article II, Section 1, clause 5, of the U.S. Constitution, and requires that the President be born in the United States (jus solis) AND of citizen parents (jus sanguinas).
The definition of natural born Citizen appears in the holding of SCOTUS’s unanimous decision of Minor v. Happersett (1874). Virginia Minor sued to be included as a candidate for U.S. President based on her eligibility under the 14th Amendment to the U.S.Constitution. SCOTUS rejected her argument and examined her eligibility, concluding that she belonged to the class of citizens who, being born in the U.S. of citizen parents, was a natural born Citizen, and not covered by the 14th Amendment. This holding has been used in 25 consequential SCOTUS decisions since 1875.
No one has the RIGHT to be President. The eligibility requirement of Natural Born Citizenship (jus solis + jus sanguinas: born in the U.S. of U.S. citizen parents) must be viewed as a means to prevent split allegiance for any President of the United States.
The Constitution does not, in words, say who shall be natural-born citizens. Resort must be had elsewhere to ascertain that. At common-law, with the nomenclature of which the framers of the Constitution were familiar, it was never doubted that all children born in a country of parents who were its citizens became themselves, upon their birth, citizens also. These were natives, or natural-born citizens, as distinguished from aliens or foreigners. Some authorities go further and include as citizens children born within the jurisdiction without reference to the citizenship of their [88 U.S. 162, 168] parents. As to this class there have been doubts, but never as to the first. For the purposes of this case it is not necessary to solve these doubts. It is sufficient for everything we have now to consider that all children born of citizen parents within the jurisdiction are themselves citizens. Minor, 88 U.S. 162.
Nevertheless, there may be an issue because Rubio's parents were not citizens at the time Rubio was born. WorldNetDaily's research shows a native-born citizen born to non-citizen parents may not be what is meant in the Constitution as a "natural-born citizen." http://www.wnd.com/2012/06/rubio-autobiography-proves-hes-not-eligible-for-vp/
There are also those who assert that a native-born citizen IS a natural-born citizen. It almost certainly would have to be decided at the SCOTUS level, if Rubio were to run for POTUS. (What the Immigration Service says is not a deciding factor.)
It seems to me to be a minor point that his parents became citizens two or three years after Rubio's birth in the United States. I guess more research will need to be done to find what was the the intent of the framers was in their use of the term "natural-born citizen". I believe in strict construction of the Constitution by ruling as close as possible to the original intent of the framers, not what a judge or Justice WANTS their intent to have been. My guess is SCOTUS would most likely allow Rubio to be a natural-born citizen for purpose of Article II Section 5 of the Constitution. This may in fact be what the framers intended, but SCOTUS is generally not strict in constructing textual Constitutional meaning, which is why I think they would probably rule in favor of Rubio.