Radcliffe was an actor before he was Harry Potter; I think it's a unfair to castigate him for trying something different. The role is strange, but I think that's why it's appealing and challenging for an actor and most 17 year old actors I know would leap at the chance to play it (horse sex nudity or not).
If the direction is good it should be powerful and not sink to the kind of 'eeww, nudity, horses, icky' level we're getting here. I also disagree with people who think that male nudity detracts from a scene; that it's all you focus on. On stage, with the lighting and the atmosphere, nudity is a costume in its own right and it's far more distracting having a sex/sexual scene where actors are wearing clothes for modesty when their characters, in that situation, wouldn't be.
Although his acting hasn't been stunning in the Harry Potter films that's probably more to do with the stilted dialogue and lack of narrative scope of the films -plus, the impossibility of portraying a character that millions of people already have a fixed idea of. I'm not a DR fan, but he was pretty good in David Copperfield -it will be interesting to see if he's matured enough as an actor to play a real role.