OP, given their disappointment and your feelings about them, why do you keep going back? They can see the son ad grandchild without you. Why put yourself through the unpleasant situation?
Have you made a point of sharing info about your xxx past with them? How and why do they know so much about your past? Prior to your getting involved with their son, were you friends with them ? It sounds as though the is a lot of unnecessary information sharing.
It is regrettable that they think of a 33 year old man as a boy. His attitude about having a child seemed pretty Kim nonchalant- " we won't use contraceptives and if pregnancy happens it happens. " TBH , that does not sound much like mature thinking and planning on the part of either a 33 year old or a 40 year old. So it is not unreasonable that his parents might have some doubts and concerns about the decision making taking place in this relationship.
You claim to be an adult woman who no longer has to tolerate demeaning behavior, but you continue to visit his parents. Why are you doing this? You must know that he and the child can visit them without you.
If you are not trying to rub their faces in the fact that you are in a relationship with "their boy, " what other outcomes are you expecting.
The do not have to like you , and it does not sound as though they are inviting you. If your partner is insisting that you should go, is it possible that he is trying to rub it into their faces that he is with you?
Whatever the motivations, it is apparent that nothing familial or bonding related is happening during these encounters. It is all just a forced interaction between people who have no liking or respect for each other.
Why bother to complain about these visits when the truth is that you can put an end to all of this by simply not going? They can be perfectly acceptable grandparents to your son without ever interacting with you, and you can be a perfectly acceptable partner and mother without ever visiting his parents.
It seems that these visits only serve to make you miserable and them regretful of the choices that their son appears to have made in his personal life . All of you are inflicting unnecessary wounds on each other. You all need to find a different sport.
They are never going to have a family type relationship with you and are always likely going to view you as the cougar who trapped their son.
You left one unhappy relationship, why in heaven's name are you wasting your time expecting these people to respect you? Chances are when they see a happy and healthy grandchild thriving and doing well , they may offer some grudging respect.