One of the most notorious works by Mapplethorpe shows a man with the handle of a bullwhip in his anus. The back of a neck can be erotic, and I suppose that it would depend on your orientation as to whether that neck was homoerotic or heteroerotic to you. Let's be straightforward. There are two objections. This is not an objection to the art, but to homosexuality, and that should be dealt with in a different arena.
Artists represent what exists and their own inner feelings through art.
I admit that I was disturbed by this photo and found it difficult to look at it. I have friends who are gay, and I have had friends die of AIDS, but this made me extremely uncomfortable. As I had more time to think, I was reminded of a time when a gay friend called me late one night in distress. He needed someone to talk to because he had just been involved with a man he loved who had asked him to participate in a type of sexual activity which went beyond his normal sensibilities. He was very disturbed and shaken by the experience and needed someone to talk with.