Originally Posted by Tonberry
I disagree. I think if you already have a partner, you should mention it before you have sex. When you have several sexual partners, they're indirectly having sex with one another and they deserve to know the risk before they agree to it.
Depending where you live, it's pretty common not to mention any partners you already have when you are dating someone for the first time. Only once the relationship becomes established you start setting the terms. In regards to sex: Practice safe sex.