My W threw out the comment that I should "get it elsewhere then!" in a heated argument about our diminishing sex life. I said "Wow, are you serious, do you really mean that? I was discussing sex with you, not anyone else. However, if that is how you really feel then I will. So are you just being nasty or are you serious? Your answer is going to dictate what happens to our M."
Pretty frank, huh? She then knew it was a deal breaker and that I would take her up on the request. What made it work? I wasn't threatening or posturing, I was serious about it. Did I really want to pursue an OW and subsequent D? No, but if she cares so little about me that it wouldn't matter to her then I am done with her anyway. Why would I want to be with someone who doesn't want to be with me? M is like poker in that if you are going for the big pot (a major change in the M) you have to be prepared to go broke (the big D is the ultimate change in the M). It sounds like you're there.
Someone had an analogy about making a grilled cheese twice a week to keep their SO happy. That is absolutely perfect and shows how little some LD/ND S cares about the M. They can't make the simplest and most natural of all efforts for their S happiness and contentment. The failure to care about their S happiness and contentment is the true problem here.
I say call them on the bluff. Rattle them a little and get them out of their comfort zone. They seem convinced we will stick with them forever no matter what. Lets challenge that certainty and see if they quit taking us for granted. You don't have to pursue OW just have W convinced that you will if things don't change.