I wouldn't say that the HIV pill is promoting or endorsing risky sexual activity, such as unprotected anal sex - it's simply providing an effective safeguard for people who are in a higher risk group, who simply want to go out and have some fun. Lets face it - sex is fun and human nature is human nature, things are always going to happen, whether a condom fails or people are being reckless and going bareback - no matter what safeguards or education you put in place, it's just reality playing out.
It's also worth pointing out that the cost of the prevention pill at £400 would be drastically cheaper than the cost of a full life-long HIV HAART treatment plan, and it almost certainly would prevent infection from occurring in many cases.
Of course, we might consider whether or not treating people with HIV is a sensible thing to do in the first place. :/
Treating sick people carrying an incurable disease to allow them to live longer may not actually be the best long term public health strategy (Particularly in poorer societies)
Of course, Cue the inevitable outrage, But just think about it for a moment.