My daughter dislikes my partner..

FBaby

OP, your daughter told you very clearly what the problem is: She is embarrassed!!!! You've said here that you didn't cheat on her dad/step-dad when you got with your present partner, just that you knew him before and it happened quickly. What does that mean really. Were you in regular contact getting closer, but you didnt sleep together so that means you didn't cheat? Or is it a case of you having lost contact, getting in touch with him again and suddenly you decide to be a couple? I am assuming that if she is best friend with his daughter, there were indeed some contact?

Could it be that your DD does consider that you cheated on her dad despite you saying you didn't? Or even if she believes that you didn't, could it be that schoolmates have been teasing her about it, saying that you were having your cake and eating it whilst still with her dad?

Kids are quick to draw conclusion and tease, and teenagers get embarassed very quickly. That would explain why her friend is ok with it (because it's not her dad/mum who is being judged), and why your dd is ok when you are all together (because it isn't so odd for best friends and each parent to get together as it obvious that they are a couple when her friend isn't there).

I don't think your DD dislike your partner at all, and that is really good news, she dislikes the fact that he is with you as a couple, that's very different. I think her behaviour is a mix of her being a typical teenager and her feeling uncomfortable with the situation. I think you need to do some more explaining, something that she can give to others so that the situation as she can describe it is not so embarrassing any longer.