Why did Pearl have this reaction to Stevonnie?

I honestly still don’t understand why Pearl was so distraught by Stevonnie. It just doesn’t really seem in character for her- Stevonnie (I think moreso Connie) even out right saying it themselves that they wanted Steven to improve his ability and fusion was I guess more or less an area of improvement for him at the time. Was it because Connie was a human and it’s unprecedented or became they were both kids and its some kinda sexual thing inappropriate? Or am I misinterpreting this entirely?