The idea of a "slippery slope" is rather fatalistic. And it seems like you picture it only being slippery if their is another woman involved - which is a bit confusing.
The secret to successful swinging is communication. If you have boundaries and talk to each other - before, during, after - then the slope is much reduced to the occasional gentle grade. Swinging does not have to "morph" into anything that you do not want it to. Sharing a couple is not the first step to your husband going to hedo alone with some drunken floozy - actually, I think the first step in that direction is getting an earring when you're over the age of forty...
Spoomonkey