Well it seems like he is apprehensive and slightly threatened. If might be a good idea to tell him that eventually you will want that in your relationship with her but for now you are okay with starting off slowly and seeing how it goes. That his pace is important to you and that you wish to make sure that he is as comfortable as possible with this process. I would add at the end that you will be considering everyones comfort as much as possible and if that means that the sexual aspect need pushing to ensure that, that you will be advocating for moving ahead in this department. I see no reason to sugar coat or make promises that are impossible to keep. After all, part of a partnership is often sexual. If that is something you and her desire then its important to be honest about your compromise for him and your willingness to compromise for now.
This is reminding me of my journey with this with my partner Mono and my now ex, Leo. You can read about it in my blog from Christmas 2010 until the spring of 2011. It was a ride that I think ended my relationship with Leo. Neither of us were getting our needs met I don't think and as a result I was just not able to go to the depth that I am accustom to and really need.
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