Am I wrong to say HELL NO!!!!

My SO has a 9 year old daughter. She is the sweetest girl. We get along very well and I love her so much. My SO made plans with his ex to go on regular dinners just the 3 of them. So that there daughter can interact with both together. My thing is my SO will not have it if I did the same with my son and my sons dad. He said he does not understand y I'm so upset about it. But when I said well I will do the same he said absolutely not what is the difference