To the question in your headline- it sounds as though you are poly, although the insecurity is possible.
However, I disagree that what you had with Sebastian was imposed upon him. Sebastien said you could have sex with women. You discussed it, he gave hte green light- I don't see how that imposed it upon him. He could have said no, he had that choice- but instead he agreed that you could do it. The only person I see who's been wronged here is the girl who thought it was more than sex when it wasn't- and that's only if you lead her to believe it was.
You should try talking to your boyfriend about why he got jealous- see if you can work on that and fixing the problem while you're both calm. Jealousy is a big deal, and hard to handle, but many people can get past it. I've seen sites aimed at poly people for dealing wiht your jealousy that you might be able to show him. It's not an overnight hting, it' doesn't fix itself immediately, but if you can get to a point where he genuinely is okay with you being with other people (women or not), it might work out.