12 is an awkward age. None of us like to think that our parents have or, indeed, ever have had, sex (except for my mother, obviously, who had five children despite being a virgin).
Your DD is just on the cusp of her own blossoming sexuality, and it must be very difficult for her to accept that you are sexually active. If you were still with her dad she could conveniently ignore this, but when your B/F is stopping over it can hardly be just because there is nowhere else for him to sleep.
I think that you are right when you say that you brought him to meet them (and stay over) a bit too soon, but what's done is done. However, while I agree that you can't let your children dictate your life, you really do have to put them first. She may be feeling very threatened (not sexually- just that she is going to lose first place in your affections; she is still very young and is a welter of conflicting hormonal emotions); she may also feel that you/she is betraying her dad (I know this isn't logical - but emotions aren't).
Could you put off the camping holiday for this year? If you are both still together next year she should be a lot more comfortable with him. Can you perhaps share time together on family days out etc, where she can keep her personal space (tents are pretty intimate, even when they are big ones) and there are other things to occupy her time.
She is young and must be feeling very upset about your relationship. She may feel embarrassed, or threatened, or just "teenage". Cut her some slack - if your B/F really cares about you he will be prepared to take his time to form a proper relationship with your children as well as with you. If you force him onto the family, she, and your other children could well become resentful and difficult, which in itself could threaten the relationship.
As surlycurly has said - Kids come first.