If one is innocently unaware that one has a communicable medical condition, then this would be a plausible excuse (here by "innocently" I mean to exclude cases where one has recklessly ignored obvious symptoms). Another excuse might be that the communicable condition is very minor (which I believe herpes is not) -- a slight itch that disappears naturally and permanently after a few hours, for example.
The fact that the other person hasn't asked might be an excuse in a social environment in which only very few are uninfected and in which everyone takes for granted that those they interact with already have the disease ("how could I possibly have known that you are one of the 0.1% of uninfected people; you should have told me this, at least if you wanted to remain uninfected!"). This is obviously not the environment we're in.
Being romantically involved goes along with an expectation of love or at least care and concern for one's partner. Given this cultural context -- which may not exist everywhere and in all subcultures, of course -- it is reasonable for one partner to expect the other to disclose risks and dangers the latter might pose. The romantic context would be considerably disturbed if the burden were placed on the former partner who would then have to think about all the possible risks and dangers she might be exposing herself to and ask about each of them individually. If your date confronted you with such a long checklist on one of your early dates, this might well disturb the romantic mood, and you might well say part-way through the interrogation: "Look, I care about you, and if I posed a danger to you in any of these ways, then I would surely tell you or find a way to neutralize the risk." When this would be a natural thing to say, it indicates that we take responsibility for disclosure to lie with the partner who poses the risk rather than with the one who would be endangered by it.