Hi Tonberry - I am a mono wife married to my poly husband. We have been together for 19 years and married for 14. He has had a girlfriend for a year. Prior to falling in love with this woman - he was mono as well.
There is a certain sexual act - that I have only done with him. He has done it with other woman prior to, but not since, me. It is the one thing that I have asked (not told) him to please not do with her. To me, it is something special between us that I would like to hold onto. They can do ANYTHING else - positions, toys etc - and I am ok with it. My husband is very resentful about this - even though they can do 99% of other things - he is holding onto this 1% that I would like them to not do. He had even threatened, but not followed through, to not do it with me as long as he couldn't with her.
I understand that for some, maybe not all, but some poly people do not think that expressing your love should be limited. That there is enough to go around. My mono-mind has a hard time with that. IF, and that's a big IF, I ever had a boyfriend, I guarantee that my husband would have some things that he wasn't able to handle and that he would ask me not to do. I would understand.
Remember - your husband did not ask for this. This is a lifestyle he is trying to accept and adapt to. IT ISN'T EASY! This is literally the hardest thing I have ever had to do!
I have a lot of insecurities and jealousies - that weren't really an issue until the GF came along - then I felt uneasy, nervous, self-conscious and I felt like I had to compete with her. I am still worried that she does things in bed better than I do. She's not skinnier than me, or bigger chested than me, - but I think she is more sensual than me. My mind wonders - and I am sure that I am making a big deal about nothing - but it's still in my mind. For men - I'm sure there is the "size" issue that would make a man insecure or worried. He might be afraid that you'll prefer the other man sexually and compare them, or be disappointed when you are with him. There could be so many things going through his mind.
I think the fact that you guys are talking about this stuff now, instead of after you fall in love like what happened to me, is great! Keep it up! Maybe you could try to find a marriage counselor that has experience with polyamory - that you could both go to, together or seperate. You will find tons of great advice on here too - and there are many books that are recommended that will help you. You can do a thread search for those.
For me, education, research, counseling and communication have been key. I still struggle everyday to accept this change in my marriage! Try to be as patient as you can with your husband. He will have good and bad days - just ride the bad ones out - they always get better. Encourage him to read these posts - and possible ask questions on here himself. Also remember, that right now - this is all hypothetical. Emotions are bound to change and get more complicated once there is actually another person in your life.
Right now - while I am trying to accept this - I feel like I need extra reassurances of my husbands love and committment to me. It's normal to have doubts and insecurities about that - you just need to make him feel that he is still very much loved, wanted and needed.
Be patient - and communicate, communicate, communicate!