I have a monogamous bf that now lives with me (MonoVCPG). My blog is filled with our ups and downs. We have written a lot here on the topic in the last four years, as have others. Try doing a search for "mono/poly" and you will have a lot of information to read.
On the whole its working great for us in our relationship. Adding new partners after the existing ones he knew when he came into my life was a struggle. Casual relationships of a sexual nature has been a struggle too. Its a whole different ball game and if you don't love it, it won't work out. If you are ready to compromise for a good long time, it could. By compromise, I mean sacrifice what your nature is. It took us years to find boundaries and agree to certain things... With poly partners it took months. If you are willing to put that work in you could make it work.
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