Originally Posted by Olivier
I think you're looking at it from the wrong angle. If you meet someone new are you instantly monogamous with her? The answer usually is No. During the time your relationship evolves and you had sex together several times, I think you can make it clear to her that you are not ready for a long term relationship, but just want to keep it open for now. This is much easier, and pretty much what most people do - polyamourous or not.
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.