It may be, especially if you are sharing your sexual thoughts and pictures with someone who is under 18, and no it doesn't matter that you are a minor as well. Texas Penal Code 43.24, makes sale, distribution and DISPLAY of "harmful material" to individuals younger than 18 years a Class A Misdemeanor, punishible by up to a year in jail and a $4000 fine. So, what is "harmful material"? This is a little vague, but the law describes it as "material whose dominant theme taken as a whole:
(A) appeals to the prurient interest of a minor, in sex, nudity, or excretion; (B) is patently offensive to prevailing standards in the adult community as a whole with respect to what is suitable for minors; and (C) is utterly without redeeming social value for minors. Furthermore, the term "display" includes accidental aka "reckless" exhibition, such dressing with curtains open or leaving a bathroom stall unlocked. There are several defenses to this charge, however being underage yourself and/or having a romantic relationship with the recipient are not on the list. The only true defense requires that the sender/exhibitor prove that the sale, distribution, or exhibition was justified due to scientific, educational, or governmental interests.
So, unless you can produce your Phd. dissertation or your medical degree, it's best to watch where those selfies are headed.