There seem to me two be two issues tied up here.
The first is whether Brooke Shields has a right to have a sexually explicit image taken of her when she was ten removed from public display. I would support her right to have the image restricted, but I believe that issue has to be determined in the US. American courts have already decided against her.
That leaves the second issue whether this photomontage should be banned from the Tate Modern. I think not. The image is already publicly available, and people should be able to make their own judgment about it, morally and artistically.
What to me is the most unacceptable is to have police arriving at art galleries and threatening the promoters of art exhibitions. The police should either get a court order against an art exhibit or keep out of it.