Honesty. But before even having that conversation you need to think about the answer to probably the most common question. "If I say no are you going to leave me" because no one wants to be put in that situation. If you know the answer to that then say its something you have wanted to try. But let me tell you this is not something you try for a week decided its not for you and leave the lifestyle, because it is a lifestyle and you need to make sure that you are both ready for that before you jump in.

Exactly, I couldn't have said this better myself!

If this is something that is an entirely new lifestyle to you, and you've never done this in previous relationships, you'll also want to at least START a list of boundaries. For example, in the past when I was in poly relationships, we had a rule of thumb. Before you could add a new partner, they had to meet and get to know ALL of the other partners, and if the other partners were okay with it, it was okay. But everyone had to be included so there was no jealousy. SOME couples prefer to make sure that they do NOT meet your partner so there are rules set about where you can go to eat, stay, etc.

You'll want to consider all of this first, before bringing it up. Having a "game plan" is very important