Here are some previous discussions of this matter:
http://smnetwork.org/forum/the-hardline/do-members-need-equity's-permission-to-do-college-shows/http://smnetwork.org/forum/the-hardline/being-offered-my-card-good-idea/http://smnetwork.org/forum/the-hardline/when-to-join-equity/...and my own take on the matter, written way back in 2001...
http://smnetwork.org/forum/articles-from-the-old-site/candidate-for-life-%28or-why-i-haven't-joined-aea%29/To answer the cliffhanger from the end of the last article, no, I never joined and my rep never had the chance to snicker at me.
AEA SMs do take non-union work from time to time. I've seen members do so for educational situations or high profile international work. If you're going to be doing something that will greatly improve your skills or visibility as a union stage manager then it may be worthwhile. However, if you're only doing it because you can't find any other work, then IMHO getting your card was a mistake. You now need to make the best of a bad decision on your own in a way that does not harm the labor union that you joined.
I never joined because I was not willing to make the sacrifices required to go pro. If you join a labor union and then immediately seek to violate one of the primary principles of union labor (i.e., union talent only works for union shops) then you have misunderstood the entire concept of organized labor. If your community has no openings for AEA SMs then you should have known or researched that fact before joining and done the math. The union is only able to offer you all the nice perks of guaranteed wages, insurance, safety assurances, etc because your talent and the talent of everyone in the union is their collateral in collective bargaining. You are now a bargaining chip. You certainly can walk away from that role. However, if you've any experience with unruly casts I'm sure you'll understand how difficult it is to field a successful team when the players keep deviating from the playbook.
If a producer can have your proven union-caliber professional talent at cut rates and without all the hassle, paperwork and liability of being an AEA shop, then what incentive do they have to switch over to equity contracts at all? How can AEA grow as a presence in the theatre industry if its members are deliberately undermining its power to negotiate? You may think, "oh, I'm just one person, my presence or absence won't matter," but the whole point of joining the union is to say that you will make the sacrifice of limiting your work opportunities in favor of the group succeeding as a whole. Every single person who has joined AEA has faced your dilemma. The union only survives because they have, for the most part, continued to make that sacrifice.
Also, think back to when you were non-union. How would you have felt if AEA premium talent was coming in to snap up all of your gigs? It would be kind of like what happened to the Olympic Games back in the 90's when they decided to allow professional athletes to compete. Non-union doesn't need big city fancy schmancy AEA stage managers coming in to tell them how it's done in the big leagues. Part of the charm of non-union is the unconventional spontaneity that arises from not knowing all the ropes.
You made a very adult decision in joining the union. Even if that choice was not made with open eyes, it would be best to continue with the adult decisions going forward.