I certainly don’t want to accuse your guy of being inappropriate when I have so little information. On the other hand, you are her mother and you don’t feel right about something you can’t quite put a finger on. That’s something to pay attention to.
I think you should trust your instincts. You use words like “fixated” and “attracted.” He’s holding her down physically and seems to enjoy spending time with teenaged girls more than with his own young son or with other adults. It makes you repeatedly uncomfortable. So, no. You are not being overprotective. You are being reasonably cautious about possible overinvolvement of a man who is not related to her and who doesn’t seem to know his proper role in her life.
Girls that age need to feel attractive, yes. But they also need to feel absolutely safe with the male adults (father, uncles, older brothers and mom’s dates, etc.) in their lives. They want male approval. They may even try out flirting. But that doesn’t mean that they are provocative or seductive. It means they are trying to figure out how to relate to men appropriately. It is up to the adult men to give her feedback about what is and isn’t appropriate and safe.
If your boyfriend wants to be part of her life, it’s time he stops trying to be a playmate and starts asserting boundaries that are appropriate when an adult male is around an adolescent girl. He can tell her she looks pretty. He can tease a bit, as long as it doesn’t embarrass her. He can help you think through whether she is ready for camp. But he can’t be stimulating her or himself with physical play.
As you pointed out, your daughter is kind of trapped when he interacts with her. Instead of treating her like he treats you, his adult sexual partner, he needs to show her fatherly respect and teach her how to assert her own boundaries when with someone who has more power than she does. That’s what is going to prepare her to manage difficult situations with men when she is out in the world.
It’s past time to have a frank talk with your boyfriend about his behavior. Yes, it will be uncomfortable. But you have the right to be concerned. It’s your responsibility to make sure he doesn’t cross a line.
I wish you well.