As I said, my husband and I started off open. Maybe we weren't technically single as we were each dating others. But we didn't consider either of us were "joining" an existing relationship since I didn't know his other gf and he didn't know my other bf. The distance factor made that simply impossible. And neither of us saw our others very often. In the year we were open each of us saw our others maybe two or three times for no more than a week at a time. So we may as well have been two singles forming an open couple. Monogamy became a choice later, just as poly became a choice again after that. But we started open and fell in love in that way and we could have remained that way had we chosen to. Of course, we didn't know that at the time. It would have saved a whole lot of drama.
If you want a relationship that is to remain open permanently, just be honest about that fact from the beginning. You may only start with one other person. Yes, it might be a mono relationship for awhile if neither of you meet anyone else you're interested in, but if the openness is established at the start, when someone else does come along you should both be free to explore-so long as you're honest when the time comes.
Edit: IMO if you start a relationship based on a mutual caring for one another, or for another or another
it isn't swinging. Swinging is looking for specifically sexual relationships right from the beginning with little or no emotional connection and no desire to form one.