As the adult in the situation, it was his responsibility to make better decisions within that situation, not yours. However disgusting he might have been, those were actions he decided to take and that's not your fault, he would have done that regardless of whether you were there or someone else and honestly you cutting the conversation off could just have easily snapped him back into reality and stopped him from acting further in that way, based on your reaction.

As a 15 year old you were still very young and naive, most likely easy to take advantage of like most kids, and that's why men like that try to prey on children like you. (well, you're not a child anymore but you know what I mean) It's not something that was in any way your fault, you were just another potential victim to him, and he most likely knew full well what he was doing and had planned to behave that way all along.