Selfish or else just ignorant.
As much as it blows my mind in this day and age, a lot of people just aren't well educated on safe sex and STDs. I don't know if he realized he was putting her at risk, or if he sincerely believed the lover was safe to have unprotected sex with. We can assume that he himself did not want an STD either.
After all, they started out with condoms and only quit after she tested negative. He's naive enough to believe he's her only affair and that her husband is also faithful.
Now if all those variables were true, then it would theoretically be safe to be fluid bonded with her, assuming other birth control is in place of course. However I see no reason to believe that she's not having other affairs. If your boyfriend already knows you're married, it makes it extremely easy to see other men, you just say you're with your husband.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker