If there is an HIV+ man who identifies as "always & truly exclusively "top", then I would suggest while he may claim that role... I suspect he tried the other position at least once and it only takes once.And as a man familiar with the story of immaculate conception and believe there is only one miraculous insemination. Therefore consider if he is having M4M sex he surely tried all the options before deciding his preference. Well that is how I see it but we all can differ in that opinion; but as I originally said "I also think it demonstrates a lack of character when an individual does not share information that the partner may want to know."
When you stumble, make it part of the dance.