Anyone wishing to or determined to enter into a relationship with someone of a different HIV status obviously should evaluate the totality of the situation according to their own set of circumstances and attitudes. Just because a difference exists does not mean that you have to jettison any possibility of a relationship; even an exclusive, long-term relationship, if you educate yourselves about practices and treatments. There is no absolute right or wrong answer to this question. The only absolute is to not let ignorance or irrational fear dictate a relationship decision.
I have known literally dozens of HIV-discordant couples, and within my particular set of acquaintances there has not been a single case of a partner seroconverting after the relationship began. I have, however, known of several couples that were, or believed they were, HIV-negative where one or both of the partners seroconverted after the relationship began. Maybe the former group was more carefully cognizant of facts they knew to exist, and exercised care accordingly.
There has been a lot of discussion since the Vernazza study (commonly called the "Swiss Study") came out a year ago stating that not one case of seroconversion occurred among heterosexual serodiscordant couples having unprotected sex, ostensibly to get pregnant, where the seropositive partner was on HAART and had (1) an undetectable viral load for at least 6 months and (2) no other sexually transmissible disease(s). [Vernazza P, Hirschel B, Bernasconi E, Flepp M. HIV seropositive persons without sexually transmitted diseases under fully suppressive antiretroviral treatment do not sexually transmit HIV. Bulletin des medecins Suisses 2008; 89:165-169.] This was exciting news, but no sensible person would conclude that it means the risk of transmission is zero. But for many, if not most, couples it describes, the risk may be extremely low, and may be reduced to near-zero with sensible precautions.
One's criteria for deciding on a life partner may well be different from those for choosing casual sex partners. Love is a still a perfectly valid factor to consider even when HIV is an issue.