I am HIV-negative and starting to date someone who is HIV-positive. I would like to pursue a relationship with him, but only if I can be 100% safe 100% of the time. Is this possible? If not, should I break things off?
Whenever an HIV-negative person is in a sexual relationship with someone who is HIV-infected, it is not possible to be 100% sure that HIV will never be transmitted. However, a lot is known about how this virus is transmitted and consistent and correct use of protective barriers can reduce the risk of transmission to a very minimal level.
There are many resources available on the web and elsewhere that provide detailed information about safer-sex practices. I would strongly encourage you to take the time to educate yourself about the facts of HIV transmission and prevention.
As for becoming involved with this particular person, there are many reasons as to why one person may not want to pursue a relationship with another person. There are a lot of "risks" (physical and emotional) when we make ourselves vulnerable by truly opening ourselves to another person and daring to "fall in love." I do not believe that HIV status, in and of itself, needs to be a reason to not pursue a relationship. While being in a mixed-status relationship has its challenges, it is certainly viable. There are many people having happy, healthy, and satisfying long-term relationships even when they are of opposite HIV status.
To put some of this in another perspective, there is never 100% chance we will not get hit by a stray bullet or a "hit-and-run" driver, die in a plane crash, get sick from food poisoning, be injured by falling debris when walking down the street, be abandoned by a lover -- the list can go on and on. But we do not stop ourselves from leaving the house and engaging in life and relationships. With that said, you or anyone else certainly has the right to select a partner based on whatever characteristic may be important to the person -- whether that be HIV status or any other physical, health, or personality attribute.