Today's lesson: Fellatio is a bad thing

[Orrex]

5. I don't know that it necessarily needs to be homophobic or a rape threat.

It can be, but it can equally be a generic insult along the lines of "commit some act to humiliate/injure/diminish yourself for my benefit/amusement/satisfaction."

Seem unlikely? Well, the language abounds with such idioms. For example:

"Drop dead."
"Go to hell."
"Go soak your head."
"Go jump in a lake."
"Eat shit."
"Kiss my ass."
"Get stuffed."
"Go take a flying leap."
"Go fuck yourself."
"Go take a long walk off a short pier."
"Get bent."
"Eat my shorts."
etc. etc. etc.

All of these have more or less the same meaning.

To concentrate solely on the perceived sexual connotation of the idiom is to pursue one possible interpretation to the exclusion of all others, with little clear justification to do so. The purpose is to deride the recipient. The particular mechanism is a function of societal norms and colloquial tradition.