Kerry Lynn Hanson, 48, was convicted of the felony charge, which carries up to 15 years in prison, in St. Louis Circuit Court, as was his girlfriend, Angela Marie Niemczyk, The St. Louis (Mo.) Post-Dispatch reported.
Jurors found Niemczyk guilty of the charge because she knew Hanson was HIV-positive and suggested a foursome in which Hanson and the 18-year-old would have sex at the Mardi Gras party on Feb. 13, 2010, the Post-Dispatch said.
Lawyers for Hanson, a manufacturer's representative from Lake Saint Louis, Mo., and Niemczyk, a beautician from St. Charles, said they plan to seek a new trial.
The 18-year-old woman did not contract the HIV virus.
Lawyers for Hanson and Niemczyk suggested the 18-year-old woman lied about having had sex with Hanson to get back at her boyfriend for making sexual comments about Niemczyk.
William Goldstein, a lawyer for Niemczyk, challenged the 18-year-old woman's credibility, saying she admitted drinking nine beers and using hallucinogenic mushrooms. Goldstein also said the 18-year-old and her boyfriend gave inconsistent versions of what occurred that night.
Goldstein said Niemczyk has dated Hanson since summer 2009, about the time he found out he was HIV-positive, and that she is not infected.
Art Margulis, a lawyer representing Hanson, said the 18-year-old had learned days after the encounter that Hanson was HIV-positive but did not get tested for weeks and went to police only after her boyfriend threatened to do so.
"What person, under those circumstances, doesn't get a test as soon as possible?" Margulis asked. "Our position is such exposure never occurred."