Why is the middle finger so offensive?

Answer 1

The "one-finger salute," or at any rate sexual gestures involving the middle finger, are thousands of years old. In Gestures: Their Origins and Distribution, Desmond Morris and colleagues note that the digitus infamis or digitus impudicus (infamous or indecent finger) is mentioned several times in the literature of ancient Rome. Turning to our vast classical library, we quickly turn up three references. Two are from the epigrammatist Martial: "Laugh loudly, Sextillus, when someone calls you a queen and put your middle finger out."

(The verse continues: "But you are no sodomite nor fornicator either, Sextillus, nor is Vetustina's hot mouth your fancy." Martial, and Roman poets in general, could be pretty out there, subject-matter-wise. Another verse begins: "You love to be sodomized, Papylus . . .")

In the other reference Martial writes that a certain party "points a finger, an indecent one, at" some other people. The historian Suetonius, writing about Augustus Caesar, says the emperor "expelled [the entertainer] Pylades . . . because when a spectator started to hiss, he called the attention of the whole audience to him with an obscene movement of his middle finger." Morris also claims that the mad emperor Caligula, as an insult, would extend his middle finger for supplicants to kiss.

It's not known whether one displayed the digitus infamis in the same manner that we (well, you) flip the bird today. In another of his books Morris describes a variety of sexual insults involving the middle finger, such as the "middle-finger down prod," the "middle-finger erect," etc., all of which are different from the classic middle-finger jerk.

Answer 2

It may be because ...being the largest finger of all others...it represent an erected p---s. So an offensive one.
It is a very old thing.