Plan it, organize it, pay for it, but then leave…fair?

My fiancé shares custody of his 10 year old with his ex-wife: (half the time with her, half with him.) We have been together 2 years, and I have a fantastic relationship with his son. His ex-wife (who left him for another woman) does not want to have a birthday party for him as she thinks it will be too much work (and too much explanation as to why she is divorced etc.) The little boy is desperate to have a party. I offered to have a party, which was welcomed by both fiancé and ex. I’ve spend almost a month planning this thing, and 2 days before am told I have to keep “a low profile – to the side if you can” in the event that people “get confused who I am” Both fiancé and the ex, have told me (like I don’t know) I am not the boys mother, and that I have no right to be a center part of his birthday party! Onlt his real mother should be. I know full well that I am not a parent to him, but considering I planned, invited, coordinated and paid for the entire party, I think I should at the very least be allowed to partake in some of the fun. I am not asking to be made: “queen of the night” I just want to enjoy the party! Am I being unreasonable, or is this how things are with kids from divorced families?