I'd say there is a moral imperative on everyone to know that they aren't carrying HIV before they embark on any behaviour that might entail passng it on, and a moral right of everyone to know that's true of the other person.

Whether that should be backed up in some legal way is another thing, and probably not - but it is reasonable that anyone who infects someone else with HIV because they hadn't ensured that that wouldn't happen should be liable.

Everyone. In our society it may be predominently gay men who are at risk, but of course that isn't the case in many other places where patterns of behaviour are different.