It's a really creepy question.
In American criminal jurisprudence, before a person can be convicted of a crime, the prosecution must prove that the individual possessed what is called mens rea, that is, a "guilty mind" or "criminal intent." That is, it must be proven that the actor acted with intent to cause the harm. There are also other causes of action against an individual that require a mere "recklessness" about one's actions.
Much of the law is still undeveloped in this area ... many state legislatures (which write criminal law) still wrestle with this very question.
From a strictly "ethical" and/or "moral" (as opposed to "legal") viewpoint, it is, in my opinion, inescapably abhorrent, irresponsible, and wrong that an HIV-positive man would knowingly have unprotected sex with another man - even if the non-infected partner "consented" to the sex and with full knowledge of his partner's status.
Our laws convict a young father when he intentionally smashes the skull of his infant child. To me, the act of an HIV-positive man knowingly spreading the HIV virus to others is equally culpable.

Hope Springs 2010 WoR Alumnus
"I'm here, and I'm on the mend."