Why are my boyfriend's mothers and sisters always threatened by me?

I have dated three different men and each relationship becomes strained by a dislike my boyfriend's mother and sisters end up aculating for me. Then shortly after we end up breaking up. At first I thought it was a spout of bad luck, but now I'm wondering if it's something I'm doing? I have to say that every man I have dated, his mom is either a single parent or my boyfriend has been a 'mama's boy'. Is it just my choice in men, or am I doing something wrong? It starts out that the mom ends up not liking me, and then the sisters follow their mother's footsteps. What's the deal? I am nothing but polite and well-mannered, and I'm not two faced. I only have nice things to say about them. They just start bashing me for no reason via text messages, e-mails, and phone conversations to my boyfriends, but then are polite and nice to my face. What's going on here? Any POSITIVE advice would be greatly appreciated.