Seems like a tough situation for you emotionally. *hugs*
I think you need to take a bit of a step back. I see that you feel very strongly that his relationships are his to manage, and you don't want to affect them. You don't want a say in what he tells his sub and you don't want to affect the DADT in his marriage. The relationships are separate entities, and as long as you are happy in yours, the rest is his to manage. Is that correct? If so, I interpret it to mean that you have little influence/power when it comes to his other relationships, and you want it so. That is okay, and a valid place to be.
But why is it then that you worry so much about what you might cause to happen in the other relationships?
With power comes responsibility. If you have power to influence, you should use that wisely, and you should be concerned about more than yourself.
But you don't have/want power to influence. If you want to let him manage his relationships, let him do the work of managing the relationships. You focus on you.
Right now he is having his cake and eating it too, at least when it comes to your relationship (and by the sounds of it, at least with the sub as well). He gets to do whatever he wants, and you have/want no say in it, but then you worry about the consequences. Doesn't that seem messed up?
You are not responsible for his other relationships going well or going badly, especially if you have/want no influence.
If you ask for what you want, you aren't directly causing anything. Do you see that? He has choices to make. He has chosen to be in multiple relationship. That means that sometimes he may have conflicting commitments. He is responsible to manage that, not you. He also has a choice if you ask for something. He can choose to say yes or no. If what you ask for is something that will affect his other relationships, he needs to take that into consideration, and answer you accordingly. You don't need to pre-emptively worry about that. You aren't responsible for his other relationships. But you need to be true to yourself to be happy.
Are you sure part of your motivation in not asking what you want isn't coming from fear that if you do, he will say no? Fear of finding out just how much (or little) of a priority you are to him and how much (or little) he cares about you?
I can totally understand that. It would suck to find out that he doesn't care. But doesn't it also suck being with somebody who doesn't care and not knowing that? Or being so afraid to ask that you never get what you need and end up miserable, even though had they known about it, the other person would have been happy to do it?
I have a hard time asking for what I want, too. And most of the time there is fear in the way, that's why I'm suggesting it. It can be fear of abandonment. For me it's fear of not being accepted/understood as I am. Anyway, the first thing is to admit the fear, and the second is to face it. It's really a choice between misery and potential for happiness. You can sometimes even find that what is making you miserable isn't in fact not getting everything you want, but the fact that you can't express your feelings/wants to your partner. I.e. sometimes you feel better for speaking up even if the other person says no. Other times, it's a dealbreaker for you, and you have some difficult choices to make. You might still feel better, though.