Might be time for a random post for me, in which, I daresay, I'll spout utter bollocks!
OP, I perceive you and DH are now in a game of brinkmanship. On the face if it, this is an argument about sex. But it's not. Its about control. You have both over I guess months and years been outlining your positions. You know where each other stands.
Getting to the point in a relationship, and I've been there, and saying I need it to be open, pretty big shit.
You're frustration both sexual and intellectual is shining through your writing.
So in the game, who moves first? I cannot answer that for you but what I would really recommend is that you don't do it covertly. If you want to take a lover, tell him but be aware that it could tip him over the edge.
I say dobt lie or be discrete, not just because its ethically wrong bit this shit always gets out and I guarantee, years down the line, it will be used against you.
And
because wanting and needing sex and intimacy is vital.