I made that mistake once, too. So I'd not tell unless it is a really serious relationship. Date and have fun. Have (safe) sex and have fun. When you meet 'the one' I'd be more inclined to tell, but not until you know for sure.
I wish there were a way we never had to tell, but the way life is, it is better that your significant other hear it from your lips and not some well-meaning friend or relative.
I told my SO on the third date as it was obvious that he was very interested (as was I) but wanted to get it out there before any physical stuff happened. It was a non-issue. Until then I couldn't allow myself to get excited, because I knew telling could end it.
As far as not telling others, I have told him that NO ONE will be told, and I will deny it to my death, LOL. ie he can't tell his family and friends. So far, nobody can 'tell.'
Jay