The sin is not yours because you didn't know he was married.On his end, he knew and looked outside of marriage , which is adultery.{Unfortunately, this type of thing is going on all too often}If you move on now, I see you having no sin in this.But the prayer would be for you to overcome him and look elsewhere, because going for him knowing he's married, or even hoping for him to divorce so you can get him, then God would see sin in you.Don't combat sin with sin!That's the best I can tell you. May God help you overcome...