In Genesis 35:22, Reuben had intercourse with his father's concubine Bilhah (Rachel's slave that gave birth to two of Jacob's sons). Jacob heard about it, but remained silent about the affair. The text passes over the event like it was a minor offense, but at Jacob's deathbed, the event was brought up with a vengeance. Even though Jacob said nothing during all his years, his deathbed words, and the behavior of Reuben after the affair, suggest that Jacob held on to a deep anger that was never reconciled or brought up until he was ready to die. Even at his death bed, Jacob did not forgive his son for what he did.
As mentioned, the text only briefly mentions the affair as if it were a minor infraction. However, for Reuben it was not minor.
I believe that Reuben had no sexual drive he was satisfying with his step-mother. I believe he acted in a moment of vengeance and anger attacking his own father.
Let me clarify. When King David ran from his own son Absalom, Absalom took 10 of David's concubines (live in girlfriends) and had sex with all of them publicly so that all of Israel could see that he was dishonoring his father David. It was not an act of lust, but an act of sedition and dominance. It was an act designed to disgrace David and thus let all Israel know that Absalom had no intent on ever renewing relationship with his father. Reuben's act was very similar to this.
In their era, raping, murder, and pillaging were aggressive acts of war that sometimes spilled out into normal life. Reuben's sex with his step mom was an aggressive act of dominance over his father.
Reuben was Jacob's first born of 12 sons and at least one daughter from 2 wives and 2 concubines. As the oldest he was the leader of the siblings, the expected heir of most of dad's wealth, the one who would get a special blessing from the father, and the boy who according to culture should get most of dad's attention, love and respect. Politically speaking, he was to be the head of the clan when his father died.
Shortly before the sexual escapade, Reuben's father Jacob (the father of the 12 sons) faced a life and death situation. Fearing that he would lose all, Jacob divided up the families according to importance. The servants and slaves were placed in the front of the line in case there was a battle and people were to be killed. The concubines and their families were next. But when it came time for the 2 wives and their families to be lined up, Reuben discovered for the first time that he was not his father's favorite. His mother and her sons were placed next before Aunt Rachel and her son, Joseph, who was the youngest of all the children. The fact that Rachel and her children were last, showed everybody that the youngest child Joseph and his mother Rachel were his dad's favorites.
Because Joseph was the youngest (this was before his younger brother Benjamin was born), the brothers expected to see Joseph and his mom to be the least in importance. But to see Joseph and his mom set apart as the most important was an insult to all the brothers - most of all to Reuben.
Fortunately, the entire family passed through the life and death situation without violence. But unfortunately for Reuben, the damage was done. Rachel (Joseph's mother and Jacob's favorite wife) died after giving birth to the 12th son Benjamin; and Reuben who was still angry about his dad's choice of favorites, went into Rachel's servant's (his dad's concubine and mother of some of his half-brothers) tent and had sex with her. Like a warrior, Reuben felt like he was getting vengeance by acting on an impulse and lashing out in anger because he believed he was betrayed.
Vengeance seems to work in the movies, but in real life it turns the tables in unexpected ways. Rather than remaining the victim, Reuben had to live the rest of his life with regret, knowing that he hurt and insulted his father deeply. He would live with that regret, compounded with the fact that his father would not talk about it, or deal with it. He would live his entire life with regret, trying with every opportunity to get back some of his father's respect; only to lose over and over again... but then that's another story.
There is a great moral in all this. If you get angry, don't let impulse direct you...you may regret it the rest of your life.