Why did Sébastien change his mind about being ok with you being with another woman? Was it that he was not as prepared as he thought he was? Did he think it was going to lead to a threesome? Did he start worrying about you falling in love with her?
Wanting to escape monogamy is not necessarily polyamory. Maybe you are just into swinging? But since you say you want to love more than one person, it sounds more like polyamory.
You could have some insecurity on top of all of this as well. Do you see yourself pushing away anyone when you get close to another person? Are you trying to run away from a serious relationship?