Hello, I am Rafael. Thanks for asking your question - I'm here to support you. (Information posted here is not private or confidential but public).
I am sorry to know how shocked you feel about this situation.
Based on your story it seems your agreement was to be honest with each other, that you would continue in your own relationships-marriage while having a sexual-affair, but he was not truly honest when he decided to hide from you that he was getting married with his girlfriend, right?
It seems that nothing has changed for him since he is still contacting you as for the past six years, but you feel betrayed because of him not telling you about his marriage. Did you fear he was going to change in your relationship because of his marriage? It seems everything would be the same, since you were just together a few weeks before he got married and now after a short period he is wanting to be with you as before.
The consistent action would be to confront his behavior, the fact that he betrayed your trust when not telling you bout it, and see what he has to say about it.
Then depending on your needs and expectations about this relationship, you would assess if this is what you still want or not.
Whenever a person does something that appears unacceptable, dishonest, abusive or inconsistent, confronting such behavior is always the best approach, otherwise you would be codependently enabling the very behavior you feel was wrong and inconsistent with your agreement.
You need to be very clear about what you want from him, and dialogue with him about it, to see if he continues to meet you with his own expectations, as it seems to be the case, and if you feel his reasons for hiding it from you are good for you, then you would choose from there if what you want is to continue or not with your relationship.
Right, as you said, you have been having a sexual affair for this long, while each having a spouse-partner, and you did not mention that your plan was to leave your partners in order to be together with time, in an exclusive relationship.
Then your reaction could show you have been fueling the expectation this affair could evolve into something else? If that is the case, you need to bring yourself back to reality and remind yourself of your agreement, and if you still feel comfortable with it and willing to afford what it takes you would continue, otherwise you would end it.
You're very welcome.