It comes after a 22-year-old man was cleared of the same charge after the court was told he had not had sex with the girl.
The judge said: "There is a real risk of the jury inappropriately convicting one or more of the defendants upon the simple basis of their admittedly wholly disgraceful conduct, even if the finding of the jury had been that consent was given by the complainant."
The acquittal comes after the girl, who cannot be named for legal reasons, spent six days giving evidence from behind a screen. She had lied about her age, claiming to be 17.
Forbes, who is on loan at Grimsby from Queens Rangers, said: "The judge took the unusual step of throwing out the case against me at an early stage.
"Having heard the evidence of the complainant in the case, the judge found it to be so flawed, weak and contradictory that the prosecution were not allowed to present their case to its conclusion.
"The last nine months have been hell for me and my family but I want to thank everyone who has supported me throughout.
"My reputation has been attacked through the last few months. I wish for all to respect my vindicated character and reputation.
"The result today allows me to continue playing football, which is all I ever wanted to do."