Sorry, BoyF, that sucks, family like that.
I'm having a little trouble understanding why you would care what such people think of you? Do either of your girlfriends have families you could adopt? I absolutely adore CBFs family, and they have claimed me, regardless of whether or not I'm his girlfriend (which I am no longer). I love FBF's mom too, though she's on another continent and I don't know her as well. What I find amusing, tho, is that CBF loves my family; we are each not so crazy about our own families.
Anyhow, if my family treated me as badly as yours seem to do, I would quit them and quick. Now, perhaps they're putting you through school and you have some obligation. In that case, I don't think you need to say anything to them about poly.
In fact, if they're that obnoxious, I would recommend against saying anything. If they always think you're lying, why waste your breath?
For reference, I know what I'm advising isn't easy. I cut my mother out of my life for five years (maybe it was longer) because she was intolerable. I was pushed by my dad (they were long divorced) and my therapist to speak to her again; and after I did, they both apologize for pushing me to it.
I'm glad I did the (massive) work to make myself okay, and to make it okay for me to talk to her. But it was much more simple to cut her out (after I did it, it was)(it was really hard to do it at first).
Just because people are related by 'blood,' does not absolve them of behaving like decent human beings. If people aren't decent, you have no obligation to keep them in your life.