Hi Makaylakay - and welcome to the Forum! I very much relate to your situation, as my wife approached me a few years ago about opening our mono marriage so she could explore her resurgent feelings for an old college boyfriend. Ultimately, I agreed - and now have pretty much a "solidly poly marriage", with both of us having additional partners.
It is always important to advise anyone thinking of discussing opening their marriage with their spouse to realize that
just by discussing it, you will have changed your marriage to some extent - just by considering the possibility of opening up you have brought into question "your vows of monogamy" and their importance to you (and all that monogamy represents with its "one and only" orientation). It is also important as well as to realize that some marriages do not survive the transition to poly - it is definitely not a way to "fix" marital issues. I would venture that only the strong (marriages) survive this transition. However, since you have already believe that your husband will be accepting, you are certainly one step ahead already.
Galagirl talked about your segue way into the conversation. I would also add that since his best friend is poly, that might be a conversation starter.
In regard to your understandable concern about your husband feeling that you are suggesting poly because "there is something wrong with him" (a common concern), my wife did a number of things right in approaching me about opening up our marriage. With an emphasis on how it wasn't about me, no matter how much it might seem that way to me - which I will repost here:
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My wife, Becky, did a number of things very right in approaching me about opening up our relationship - even though she has not actually "studied" poly. Which was interesting, since much of what she did right is what one often sees recommended on the poly forums and in the literature.
1. Made sure that I understood that it was not about me - no matter how much it might seem that way. There was nothing wrong with me and I had not failed in any way. This was about her feelings and not a reflection on me.
2. Made sure I understood that it wasn't because she didn't love me as much as she ever had - but she believed it was possible to love more than one man at the same time without diminishing the love for either (the classic poly argument is to consider that a parent can love multiple children at the same time and still love any others than come along as well).
3. Made certain to emphasize that her relationship with Ben (her bf) would be "in addition to" - and not a replacement for - our marital relationship. And that our relationship would still be "primary" (after all we were married with a child in a home - with a mortgage, etc) - and that we could negotiate what exactly that would look like. I found the phrase "in addition to" to be particularly helpful in helping me come to terms with her desire to have a second relationship.
4. She was very patient - and did not try to push it along too quickly. We could take it one step at a time, and she agreed not to have sex until I felt I could accept the idea. And although she did want to include sex in their relationship eventually as a natural progression, for her it was much more about her feelings and her emotional involvement.
5. She was willing to talk about it as much as I needed to - no matter how long it took or how often - and to answer any questions that I might have - without any apparent frustration. She understood the need for me to process the situation. And she was very patient and understanding of my frustration and emotional turmoil at her request - very validating while still upholding her belief.
6. She went out of her way to reassure me of the depth of our relationship during this time - avoiding arguments, being especially loving and affectionate as well very open and honest about here thoughts and desires.
7. She agreed that being completely honest and transparent was absolutely essential - to whatever degree that I felt I needed that to be ok.
8. Made sure that I understood that I could also have another partner if and when I was ready.
And, I already understood from life experience that it is very possible to love more than one person at a time (acting on that is the issue) - and that one person cannot supply all of another's needs. These points may need to be discussed as well.
We also had an in depth discussion about sex when we reached the point where I was ready to talk about it - and agreed on the various parameters. This will obviously vary by couple - but we agreed on transparency and a willingness to utilize the sexual energy created by their relationship to enhance our own (as discussed both in "The Ethical Slut' and in "Opening Up".) This proved to be helpful when the time came - as I discussed in the comments just prior to this post on this same introductory post thread.
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Also, here's a link to a list of some of the best poly websites - that might help in your discussion with him.
I recently had the occasion to provide some introductory information to a college student who was doing a term paper on poly. So, I directed her to this forum, a few websites, and a selection of books. I shared the first five of these web sites with her - being websites that I have used...
polyamory.com
A book that a number of us recommend for couples opening up a marriage is:
Opening Up: A Guide to Creating and Sustaining Open Relationships
by Tristan Taormino
Best wishes for a successful conversation! We will look forward to hearing your story as it unfolds.
Al