You're not wrong to want to keep the relationship with your boyfriend. It's hard to say what the best thing to do is, but I was in a similar situation not to long ago, and I couldn't have ended it with my guy. When one relationship is falling apart, the idea of ending a good relationship is just completely horrible. Fortunately, my husband understood that even in the worst of it, even though he didn't like my boyfriend at all, and he never asked me to end it. We worked through things in counseling, and really started putting a priority on time together. When it was warmer, we took a walk every evening, now we find something to do indoors, whether it's take a bath together or go shopping or just sit down and talk for a bit, and then we go to bed together. It sounds trite, but we went from being miserable and wanting a divorce (well, we were conflicted, and sometimes he wanted to leave and I wanted to work things out, and sometimes it was viceversa) to really thinking this can work for the long haul over a few months. Once my marriage calmed down. *then* the boyfriend left. I explained that in the other thread I started, but part of me thinks he might have been waiting around to see if we'd divorce and then ditched when they didn't look likely (still bitter over here).
I definitely agree that it sucks that you've been put in a position that you have to choose between two people. You'd think poly would prevent that, wouldn't you? I definitely know it's possible to repair a marriage without ending other poly relationships, and there's no guarantee that ending things with the boyfriend would save the marriage. Or that ending things with the boyfriend won't make you so sad and angry that you have no energy to work on the marriage. The problem here is that your husband doesn't see things this way. He might be at a place right now that he can't work on the marriage while you're still seeing the other guy. The problem really isn't whether you should end a secondary relationship if the primary is in trouble, the problem is agreeing with your primary on what to do. I would try to make him understand your perspective, and see if he can work on the marriage while you're still with your boyfriend. If that won't work, clarify what you and your husband will do if you break up with your boyfriend, because it's not going to magically make anything better. What sacrifices would your husband make? Will he come home from work earlier, do things with you that you want to do, do more housework, etc? I'm sorry that I'm just thinking of stereotypes now, but think of what he can do to help the marriage from your perspective, and make a plan. The last thing you want to do is end a good relationship just to see the relationship you still have stay crappy or even get worse. Make sure your husband understands that fear, and if he still insists that you end it make sure your husband is committed to doing whatever he can to save the marriage.