Adults have sex for a myriad of reasons. Sometimes it is out of anger. Sometimes it is to win someone's love or attention. Sometimes it's just to get off, and, of course, sometimes it is to procreate. There is no right or wrong reason to have sex if both parties are willingly engaging in the act.
Sexuality is biological.
Many of us know those morning-after feelings of regret or elation. It is ridiculous, not to mention impossible, to legislate a person's motivation for having sex, regardless of a person's HIV-status, and that seems to be the crux of this particular case, even more so than the HIV status of either party. I wonder where the criminal activity actually springs from. Consensuality implies complicity, and the burden of responsibility falls on each individual to act with integrity and with the maximum amount of knowledge available to them. I don't know that it is possible to legislate personal responsibility or integrity.