I agree with RP on this one. To be fair, last year I was burned pretty badly in a secondary relationship where the metamour was less than enthusiastic.
If your metamour deep down doesn't like you or want you to be with their spouse, even if they won't admit it, it's probably not going to go well for you. And if ultimately, you need more than they jointly are comfortable with, then it's not a good match. I remember that sometimes I would ask my BF at the time if something was ok. And he would often forget think of the question in reference to what his wife wanted. He'd be like well, I'm fine with it. And I would have to prompt to hear what she might think. And often he didn't know. HUGE RED FLAG.
Often if the spouses are worlds apart regarding poly, you can be getting yourself into a pickle. If that is the case, then they have a lot of work to do before they can have good poly relationships. ESP, if one of the partners doesn't really want to do the work in the first place. Now, this may not be what's going on with you. But be careful. Don't be afraid to demand equality as explained by RP. I remember also that I once told my former BF that I felt powerless in the relationship (negotiationwise) and he told me that I had the power to leave (and was being serious) (oh, and said it like it was some kind of boon to me). Don't settle for that kind of relationship.