Originally Posted by insanity
How can I explain to him that beteen me and him nothing changes? (without bringing terms like polyamory, another partner, etc...)
why would you want to?
I suggest reading some threads on jealousy and having him look at them too. You can find them by doing a tag search.
I can't see how this going on and a wedding is going to ease your stress. Weddings are super stressful and so are poly dynamics. I agree with Dinged, call it off until this is sorted out or take a break from the girlfriend. Do you want this marriage to work out? It means a deeper level of commitment. You having threesomes just to make him feel better is not going to keep you together. Working on his jealousy and getting into a routine where you have alone time with her away from him is going to keep you together in my opinion.