I agree with what jessi said entirely. It sounds like your boyfriend was following a humane impulse: being decent to someone he used to care about, and trying to be a good dad. Don't make his son the further victim of their divorce by your unwillingness to be gracious. You've got what you want: this man. Be a good winner, don't try to manipulate him by whining you "feel hurt." She is not in the past, only their romantic relationship is a thing of the past. She is his son's mother and if you stay with this guy you're going to have a lifetime of her being as legitimately connected to the family as you are, through the child, not the man.
We have two exes in the family who get invited to all the important occasions, because their kids are there. (What mom doesn't want to be with her children at the family Christmas parties?) There is no interest in rekindling with their exes, but it's nice for them to be with their kids at a party, and to see the people in the extended family that they know and like. Your boyfriend's son will have a more emotionally sound life if he gets to see his dad and mom being decent to each other on pleasant occasions. If this is just you being annoyed that when you tried to express your insecurities and lay down the law about who can come over, your boyfriend came back high-handed, sheez, let it go for the kid's sake, invite her to all the parties, and go to couples counseling about his tendency to run over you when inviting people over.
Anyway, this gal has lost everything that you say you want, and you have it. That calls for a bit of empathy or even sympathy on your part. If you can't stop being suspicious, maybe reconsider the relationship. Or, if it's really just about the control-issues part of the fight, see a counselor. Such things tend to pop up in other contexts if not worked out.
So how long have your partner and his ex been divorced? I assume he and his ex share his son?
While I do agree entirely that he should have checked with you first, and that his comment about not needing permission is a huge red flag, I think exes can be friends, and it doesn't necessarily confuse kids.
There are lots of reasons why not having two Christmases, two Thanksgivings, two birthday parties, etc., helps kids, and you can be the bigger person here and co-parent well with his kid's mom. That will only help the kids. She doesn't have to be a threat to you.
It sounds like they share kids. The example he sets in how he cares for their mother is one they will take with them forever. The example you set will, too. Don't look at it as inviting the past in, but as extending the family for the kids' benefit.
His son is going through things. He's already gone through them. His parents got divorced, and now he's losing a step-dad. How nice for him that his mom, dad and his dad's partner could be together in one place for the holidays. :)
That he is inviting her over without your permission and childishly declaring that he doesn't need permission is a separate issue. You say you've been together a year - you are already living together? Do you have an ex? How would he feel about spending holidays with your ex?
Open your heart, ask your partner to keep the communication open, and see what happens.
There's so much to unpack in this I don't even know where to begin.
First of all of two people got divorced, there was probably nothing to mend in the first place, and being she already managed to get married once more after, means your husband is right. There's no way of reconciliation there
Second, she's not his past nor their children's past. She's their mother and will always be that. And as long as your man and she share children she will be part of his life!
Third, you're in his life for a year and you think your being uncomfortable trumps their willingness to have a nice evening , only because this woman makes you insecure. He has right to be mad. Whose house is it, place where you two are living and I mean who paid for it? My assumption is that it's him and you get to choose who comes in there and when.
Fourth: don't present the situation like two dinners are such a big deal. It's not like he's meeting her without your knowledge or brings her over all the time. She was once an important part of his life and he doesn't want her to feel alone and abandoned during these events. It's very big of him and you've probably made him feel worse by your behaviour. I just hope you showed some grace during the actual dinner?
Fifth: what the F are children going to be confused about? Please someone explain that part.
I am still on good terms with my ex husband, and I am thankful for that. He and I shared a decade and tons of memories together and I still can talk to him about important stuff in my life and I can still rely on him whenever I need help. Someone I date for a year certainly won't come in the middle of that, and frankly why should they it doesn't concern them.
It can also be a cultural thing but it makes me angry to think someone would deny a person coming over for Christmas!!! One day you should be charitable and do something nice for another person.
It feels to me you're being insecure, spiteful and then trying to flip it over to "what about the children".
I'll add my two cents, also having been in a relationship where children and previous marriages were a part of it. Your boyfriend was most definitely wrong in not discussing the invitations with you. After all, relationships require give and take, and healthy communication. But if he HAD discussed it with you, you should have graciously and enthusiastically agreed - assuming you don't think he has any romantic intentions with her.
If you are going to continue a relationship with this man, his child, and his ex, are going to be in your life for as long as that lasts. Being gracious to the child's mother is going to make his son a lot more comfortable, and see you in a positive light. No child wants to see someone be "cold" to their parents. Also, the ex-wife seeing you interacting with her son is going to make HER a lot more comfortable that he's being well cared for by both adults in your household.
When I met my now-husband, we each had 3 children, teenagers down to a 9-year-old. I made it a point to be welcoming of his ex's presence, and 15 years later, we all still go to the important events in our kids' lives, at one another's house or elsewhere, and we sit together in support of the children. And you know what? It is SO easy for everyone. There's no tension. There's no burden on my husband to pick between being kind to me or being kind to the mother of his children.
(Assuming there's no evil/crazy ex in the scenario,) this is how it should be for everyone who's interested in creating peaceful, low-drama relationships.