So why is Fucking an infix? Why did "fan-fucking-tastic" spring up in place of "fucking fantastic"? Throughout the rest of English, words are modified by adding things to their beginning or end; why should this situation be any different?
I think the answer lies (1) in fucking's role as a modifier for larger constructs like phrases -- it's usually inserted in the middle, not the beginning -- and (2) in the fact that when a single unattached word becomes a phrase in itself ("absolutely!") it feels natural to treat it as one ("absofuckinglutely!").
Okay, then: why did fucking's mid-phrase insertion rule spring up?
- Clarity. "Fucking" is not just an adjective but a verb (which is to say, a participle), and adding it to the beginning of a phrase can cause confusion. (There's a shirt sold in Japan, popular among those who don't like the patriarchy, that says, in English, I'm not fucking your baby.)
- Intensification. "Fucking" is, of course, a swear word and as such is naturally said at the most intense point in a phrase to propel it to greater heights. If we extrapolate for larger structures one of the placement rules vivid outlined above, it becomes "The fucking infix is nearly always placed before a stressed word", which fits nicely.