It's immoral to spread sexual diseases to partners.
Jessica has been dating Ernie for years, but has recently been seeing Joe behind Ernie's back.
Jessica wants to stay with Ernie.
Jessica breaks things off with Joe and gets herself tested.
Jessica contracts an STD from Joe, but doesn't tell Ernie about any of it and continues to see both men.
Jessica's test comes back negative for STD's, and she is faithful to Ernie from then on.
Ernie gets an STD from Jessica.