Is talking sexually through texting before marriage considered wrong?
One of the mistakes the Pharisees made was to look at God's laws so narrowly that they decided that only what was precisely mentioned was wrong. They understood that committing adultery was wrong, but they excused thinking about it. "You have heard that it was said to those of old, 'You shall not commit adultery.' But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart" (Matthew 5:27-28). A Pharisee would not think it wrong to leer at a woman or make catcalls as she walked down the street. So long as he kept his clothes on, he would see himself as being righteous.
The problem is that sin begins in the heart. "And He said, "What comes out of a man, that defiles a man. For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. All these evil things come from within and defile a man" (Mark 7:20-23).
So while you aren't committing the act of fornication, you talk about it and describe it graphically. How is that proper behavior for a Christian? "But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints; neither filthiness, nor foolish talking, nor coarse jesting, which are not fitting, but rather giving of thanks. For this you know, that no fornicator, unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and God" (Ephesians 5:3-5). What you are doing is precisely what Paul said Christians are not to do.