This is a good point.
Although we all take risks in our lives, starting with getting out of bed in the morning, there are certain risks we can mitigate.
I ride a motorcycle. This is a known risk, and a significant one. I choose to mitigate the risk by always wearing protective gear, and never riding when angry or tired. I don't drink, so that eliminates the possibility of riding drunk. Still, plenty of risk remains. I accept it, and so does my husband. This is a choice we've made.
Having sex is a risk. Certain things can happen. There are ways to minimize the chances of these things, but there's enough stories of "Oh wow, this was a surprise" to lead me to believe that nothing's perfect. It seems like having a sexual relationship with someone with HIV is similar to me deciding to go riding without my helmet right after closing time at the bars and while tired and pissed. In the snow. With bald tires. I've just upped the odds of something going tragically wrong, for no good reason.
Just my $.02.