Manti Te'o just revealed ... he's NOT GAY ... despite rumors he used a fake girlfriend to hide his homosexuality.

Katie Couric asked Manti flat-out whether he's gay because it's the most obvious explanation for the bizarre "Lennay Kekua" hoax -- but Manti replied, "No. Far from it. FAR FROM IT."

Manti also revealed Ronaiah Tuiasosopo's full confession, which Ronaiah sent via Twitter, saying, "I completely accept the consequences to the pain I've caused and it's important that you know the entire truth before anyone else."

Katie played Lennay's voicemails -- and Manti replied, "Sounds like a woman. Incredible talent to do."