Why is it forbidden for any alien species to visit Earth in Marvel NOW? I read this on the Star-Lord wiki page. Does this extend to other Marvel Universes?
The short answer is because Freyja, the All-Mother said so.
A collective of galactic empires considered destroying the Earth because it has been at the center of galactic events more than once and yet does not even have a stellar empire to speak of.
Freyja (presumably speaking for the Asgardians) sensing potential treachery, warns the leaders of said empires she would fight to protect Earth as Odin did before her. She warns them to "be content with you have."
A bit more detailed:
The Earth has been considered a threat to the other galactic empires on Marvel Earth-616. The Galactic Council comprised of the largest and most powerful forces in the current Marvel galaxy decreed Earth off-limits due to their potential threat to life in the Universe.
The council recounts the number of times Earth has either been at the center of galactic conflict or has due to the prodigious number of metahuman beings of galactic capacity there, engaged in conflicts beyond their technological capacity.
This includes defeating galactic menaces such as Thanos or the Phoenix force and rebuffing attacks from the previously unstoppable World-Eater, Galactus. The Earth has the rare distinction of having a Watcher assigned to it. (The Watchers are a species of galactic monitors that track significant events that take place in the cosmos.)
The Council included: King J-son of the Royal Conclave of Spartax, The Supreme Intelligence of the Kree Empire (very different in appearance, perhaps a newer upgrade) Gladiator, Leader of the Shi'ar empire (no idea what happened to Vulcan), Young Annihilus (assumed replacement for the earlier Annihilus killed by Richard Rider/Nova Prime), The current Queen of the Brood, and the All-Mother of the Asgardians, Freyja (No idea where Odin or Thor are) and Y-Gaaar of the Brotherhood of the Badoon.
These screenshots are from Guardians of the Galaxy (2013) #2. These events have nothing to do with other universes including the Ultimates Universe (whose existence is in question) nor the Marvel Cinematic Universe, Earth-199999.
Marvel NOW! is a re-branding effort where numerous comics set in Earth 616, the mainstream marvel universe of the comics, are re-launched with a new #1 and usually some significant change to the comic. Amazing Spider-Man, for example, had more than a year as "Superior Spider-man", and then was-relaunched again earlier this year.
The ban on alien species interfering with Earth was due to a collective decision of the major interstellar powers to contain the out-size threat that Earth presented, given our relatively small population and low technological development. This occurred most clearly in the Guardians of the Galaxy re-launch in 2013. (The comic can be found, among other places, at Marvel's online database
It does not affect the other Marvel Universes -- either the "Ultimate Marvel" or the "Marvel Adventures" lines, the marvel cinematic universe, or the various cartoon show continuities.