The CDC's form is horribly out of date. Sexual diseases are spread most commonly by unprotected sex, regardless of the gay/straight issue. The myth that homosexual sex is the most common way of getting it is apparently still clung to by many people, and the CDC of all groups should be wary of their role in the spread of sketchy information.

I believe current data shows that the disease is most commonly spread through heterosexual contact. Why the old link to gay males? Probably because there was no need to use condoms to prevent unwanted pregnancies.