Deliberate transmission of HIV is certainly immoral, but if we make it a crime, we lose the fight against it. What will happen is that people will stop testing: if they don't know they're positive they can't be accused of deliberate transmission. Who's the most infectious? People whose immune system has totally collapsed, who tend not to be very interested in sex anyway, and people who've just acquired the virus. Who's among the safest people to bareback with? Guys who have their HIV under control and don't have any other STIs. In other words HIV+ guys who look after themselves.
Yes, a certain number of people who get a positive result get very freaked out, and though it's unusual "revenge sex" has been known to happen: psychological intervention needed here. Similarly someone who's fucking around pretending he hasn't got HIV because he's afraid of rejection needs psychological help and support to understand what he's doing.
Sexually active people who don't test for whatever reason aren't taking care of themselves: there's a host of STIs that they may also have (a reason why I serosort for poz guys), many of which facilitate infection with HIV. But they won't test because they're afraid of having to disclose that they have HIV, with all the stigma and disadvantage that brings. So we end up with reservoirs of HIV in people whose health is slowly failing (and you can go a long time before showing any sign of infection) as their viral load climbs making them more infectious.
We need to change society to remove the stigma attached to HIV, to make it as normal as having any other infection. The problem is that you generally get HIV from doing "naughty things". The vast majority of people think of gay men as being the biggest cohort of people with HIV whereas in fact more woman have HIV than men. It's literally vital that anyone who's put themselves at risk test for HIV: criminalisation is about the best way to stop that testing happening.
Worse still, in the UK all the law on HIV transmission is case law, usually using the Offences against the Person Act from the nineteenth, yes nineteenth century. Because it's case law, it can change with each case, There was recently a prosecution for deliberate transmission of herpes. As far as I remember that prosecution failed. What about those parties mothers organise for their kids when one of them gets one of the childhood illnesses so that all the kids end up getting it out of the way? Chicken pox springs to mind here because my late partner caught it from the child of a friend and when I phoned the clinic to ask what I should do their reply was "how quickly can you get here?". I was lucky: it turned out I'd had it without symptoms as a kid and had the antibodies to it, otherwise it would have cost a considerable amount of money protecting me from my partners illness.
Criminalisation opens a whole bucketful of worms, the chief one being that it's going to make us lose the fight against HIV.