Why is it so hard for me to accept my fiances kids!?

I have been with my fiance going on a year now, he is 29 and I am 20 years of age. He has three kids, one of the little girls he never has met because the mother supposingly moved away when she was pregnant. The other two are twins, a boy and a girl. He said that they came along because he had a drunken one night stand. The twins baby mamma does not like me and says that she does not trust me around her kids. I am absolutely not a bad person, she has no reason to say that! She only lets my fiance see the kids about every three months, she hardly ever answers her phone. I spent Christmas with my fiance and his kids last year, we all got along fine and I bought them tons of presents. (I don't have any kids I didn't know how much fun it was to shop for them!) But, my feelings got a little hurt because my fiance was telling them to say mean things to me to make them laugh. Later he apologized about it though, saying he was just trying to have fun. Later, the kids go home and their baby mamma decides to call my mum and tell her what all exacely she thinks of me. (Not good as you can imagine.) I told him that I want kids of my own and he said that we could have one together, so that isnt really an issue, but now every time his kids come around I just really do not want to be around, and it's like I cannot accept that he has them. I don't mind him seeing them at all, I would never keep him from them. I just don't want to be around. Is that wrong of me? Why do I feel that way? Has anyone else gone through this!