a) A person commits an offense if he intentionally or knowingly:
2) makes an offensive gesture or display in a public place, and the gesture or display tends to incite an immediate breach of the peace;
I didn't know about:
(b) It is a defense to prosecution under Subsection (a)(4) that the actor had significant provocation for his abusive or threatening conduct.
So if someone "signifigantly provokes" you, it's ok to flip them off.