It isn't weird at all if it was a party for their daughter's graduation party he probably felt obligated to do so it would have been wrong for him to have a party for their daughter taking such a huge step into the world graduating from College or High School is a big deal for any parent and he just wanted to include the ex wife, I wouldn't read too much into it and may I tell you if you love this man don't let something so little like invitation to their child's party get in between you because you are going to have to deal with the ex wife from time to time. Though he says he hates her he put that aside for his child and he should be commended for that alot of parents bicker for years and they don't realize that the child(ren) suffers. I would say put on your best face and enjoy your boyfriend.
She gave him up and you have him now, don't feel second best, XXXXX XXXXX still licking his wounds from her leaving abruptly and separating the family that takes time to get over. As far as the ex wife's family they are the child's relatives also. Try and be friendly or at least ignore her at the party don't spoil their daughter's party she deserves the party graduating from college is a bug deal and she probably worked hard. If you love this man then stick by him and give it some time. He may never stop talking about the ex especially if he says he hates her so just tell him that you don't want her to be every part of your relationship and that you know she will have to be part of his life because they have children together. He will love you for supporting him. If you have anything to add click reply.