Background: When people call something or someone "So Gay", they are often considered to be covering up their own latened homosexuality. These people also do gay things and then claim "No homo", making it ok.
By this reckoning, one can assume that if a person were to call things "So Straight" all the time, they are merely reinforcing their own heterosexuality. Hence the term "No Hetero"
Frank: Hey, hows it going Steve? *slaps steve's ass*
Steve: I'm pretty good man. *slaps back*
Frank: Hey! No hetero!
Steve: Yeah man. No hetero.