I hear you when you say if you leave, the married couple will probably break up because he doesn't want to give poly another go, but she does. But I would caution you about shouldering the burden for that responsibility. They made their own decisions when they got involved with you, and they made communication difficult, so they are kind of stuck in a bed of their own making. It's not your responsibility to save their marriage. They have to be responsible for that.
Sounds like you're being tucked away and blown off a lot. Make sure they know you're on the brink of leaving. Set a timetable if you have to. If things haven't improved by a certain date, let that be the date of your departure.
You know, a break-up might not be all bad for them. It would free them up to pursue two very different flavors of relationships, polyamory and monogamy. Could end up being something that makes them both happy in the long-run, and you could be happy too, as you'd be free to seek out a relationship where your feelings, concerns, boundaries, and needs are heard and respected.
Just a little something to add to the five good posts that have been posted so far.
Love means never having to say, "Put down that meat cleaver!"