If you were having a lot of sex with various people -- or even if you just had a quickie with one person without any protection, then having unprotected sex with the next person becomes a moral issue.
Maybe it's just the case for those of use who came to age when AIDS was finally getting some public health recognition. I wonder how much that concern has dropped. Right now it's still illegal to have unprotected sex if you know you're HIV+, but ethically, having prior unprotected sex means getting tested before having unprotected sex again. Just in case. Because you can never know if you've picked up something along the way -- not just the big one, but any number of sexually transmitted infections.
