My partner and I have been together in an open relationship for 38 years. It hasn't always been easy. We've always been honest with each other. The one thing we agreed on in the very beginning is that sex and love are not mutually exclusive. Hope I got that right. What I mean is that we both believe that men in particular are not built to be monogamous. We believe we can love each other and still have sex with others. Sex is just another bodily function like sneezing, coughing, pissing, etc. It has had some good aspects in that we've had 3, 4, and more ways which were very exciting. Since we both agreed that it's only sex, jealousy hasn't been too much of a problem. Actually the only jealously I can remember is being jealous that he's had sex with some guys that weren't interested in me but I was interested in them.
I don't know how I would approach the subject with a partner who I've already started a closed relationship with. Sorry I couldn't offer any advice. I think you have to do what is right for you to keep your sanity and live as stress free as possible.