n. A heterosexual.
On 4chan, EVERYONE is a faggot (regardless of sexuality), and a need came up to differentiate between types of faggotry. A homosexual would be a gay fag, and a fan of Japanese cartoons would be an anime fag.
I wanted him to do me up the butt, but it turns out he was a straight fag.
Originally refers to those homosexuals who are so ineffably heteronormative that they act like typical breeders, even though they are a slave to the cock (or the vag). Its use has expanded as a insult for those who differ from the mainstream in an interesting or awesome way, but can't own it or face it.
"If you really like cheesy disco music, fine, but admit it or you'll be another straight fag.