I disagree. A "normal" homosexual would find satisfaction with someone who was "clean". gaijin's point is that flirting with death is PART of the erotisicm of bug chasing.
I understand your point to be that hepatitis is a risk when engaging in watersports etc. There IS a risk. However, those engaging in these activities are seeking the thrill of that specific experience. That they could become infected probably never crosses their mind.
A very huge part of Bay Area culture revolves around AIDS issues, and after a while of "just" being gay, it probably is a little off-putting to sit on the sidelines.
Just speculating...