Originally Posted by Marvel
It isn't him I have a problem trusting.
I am sorry to say this, but it is him you have a trust problem with. Most of the things from your list of fears are directly related to trusting him or not. Every single one about valuing you enough when having another relationship is. About not risking your health. About disrespecting your boundaries. About him leaving. And so on. I have to say you should look into your fears (which are absolutely valuable by the way, don't think of them as minor or something) and look how many of them stem from not trusting him.
Originally Posted by Marvel
I think that if he found just a play partner for his BDSM needs, I would be happier, but he feels he has to have the ability to have whatever kinds of relationships develop, that he wants more than just those BDSM experiences.
I know that some do better without the intimate connection of feelings when the partner takes on a new, let's call it 'interest' outside of the initial relationship. I couldn't think of that as more positive, as I know that simple sex without emotions would never satisfy me. Is it that strange to not just want a 'fuck buddy'?
On the other hand I completely understand why you don't want your private things to be discussed outside of your relationship. As you said, he would be dating that person, not you. So he should respect your need for privacy, even if it is hard, and not talk about topics which are uncomfortable for you with third persons. Even if that third person is a partner of his.
You asked what is in this for you. Frankly: nothing, besides a happy partner. For some the knowledge that their partner can be who he/she is. They take pleasure out of his/her pleasure. The hobby comparison is not that far off, despite the risky part you are right about. But every possible hobby your husband taking on would consume money and time, not spend on you or the family. Of course sometimes some limits have to be set in regard to that, but fundamentally saying "No, never!" would seem really strange to me. Think about this situation, your husband taking on a hobby, requesting two nights per week for himself alone, working on something in the basement or garage, going out with friends to see some shows, whatever, would you be jealous of that time he spends away from home? Would you regard this time and investment as threatening towards your family life or marriage?
I know that you want to say that this is something completely different, but trust me, it is not completely. I don't know your husband, but just from the way you write here, it seems to me that you make most of your problems out of thin air. I know those list of fears. I know how they work, I was already called a champion worrier
Go over your list of fears and look hard at them. Most of them should only work, if your husband starts being a whole different person I suppose. That was how my fears worked. As soon as you really take the other person into consideration, how he is, how he reacts normally (not this completely arbitrary 'what would be if ...' person): Are those fears still realistic? Do you really believe that another person could change your husband that much? This isn't a compliment in regard to him.
Lastly: Only you can decide if you want to do this for him and if you want to explore this path. You have no obligation to do this. But if you fundamentally say "No." and he fundamentally needs this, you will have to think about further steps. Because this would end in resentment.