Lentiviruses such as HIV have a daunting challenge in gaining access to a new host predominantly through the penile, rectal, or vaginal/cervical mucosal tissue after sexual exposure. Multiple mechanisms have evolved to help prevent such infections, including anatomical barriers, innate inhibitors, and adaptive immune responses. For lentiviruses, it appears that in naive or even conventionally vaccinated hosts, typical adaptive immune responses are generally too little and too late to prevent infection. Nevertheless, a combination of anatomical barriers and innate immune responses may limit transmission, especially in patients without predisposing conditions such as mucosal lesions or preexisting sexually transmitted infections. Furthermore, when infection does occur,most often the primary viremia of the acute infection can be traced back genetically to a single founder virus. Unfortunately, even a single virion can establish an infection that will ultimately lead to the demise of the host. This review seeks to describe the biology of and barriers to establishment of systemic, disseminated productive infection with HIV after sexual exposure and to discuss the possible mechanisms leading to infection by a single viral variant. Understanding the initial events of infection, before systemic spread, could provide insights into strategies for reducing acquisition or ameliorating clinical outcome.