If I were in your position, this is what I would consider. The person I am “locked” into a relationship with is obviously someone who I care deeply about which is why I am “locked.” Since there is no ball and chain, it can be broken off by a break up or a divorce. While I empathize with you that she is more conservative with her sexuality than you would like, I would consider breaking things off with her before making a move to cheat. If she found out, she would be devastated. If she never does, I would be haunted by my actions even if I could try to find ways to justify it. And if I were to break it off to fulfill my sexual needs, she would also be heartbroken and angry, but maybe not to the same extent and not in the same way, though I may never see her again afterwards.
For many cases, having a good deal of integrity means that in some way, you lose; which comes at the cost in helping to ensure others don’t. I believe that’s worth it. For this situation, assuming I was in love with her for many other reasons, I personally would try my best to patiently hope and wait for the day where she becomes more liberal in the bedroom. Perhaps you could get her to take a human sexuality class with you at a nearby community college? Maybe demonstrate the beauty in more taboo endeavors rather than entertaining crude jests about them?
Furthermore, if I knew I was prone to addiction, I would think thrice about going. Four times if I suspected I would blow exponential amounts of money meant for other purposes.
All in all, I hope you will seriously consider my opinion and then make the best decision. Life is short and its important to live freely and fearlessly but it sounds like there is a lot on the line for you. In my opinion, the best way to make hard decisions like these in a way that you will have the least amount of regret is to decide against yourself rather than seeking justification.