› firstname.lastname@example.org 
Ever since I was nine or so, I've had unexplored dominant-role power exchange fantasies. Now they are at odds with my marriage of 20-plus years (my wife isn't into it) and my worldview/faith. I feel pretty strongly that I'm fooling myself when I think that finding a similarly-situated woman to clandestinely and mutually scratch this itch would somehow be cathartic and result in resolution once and for all, but the fantasy persists. Are these type of fantasies typically lifelong? Do they wane with age?
Is it national S-M month or something? Shouldn't I have been flooded with gay, lesbian, bisexual, transexual, and questioning questions all June instead? I do like a good S-M question, of course. I was just wondering.
I doubt you experienced those childhood urges as "dominant-role power exchange fantasies." I guess, rather, that you really enjoyed playing pirates, but only if you got to tie the prisoners to the mast and do weird stuff to them, and you never wanted to be the prisoner yourself. And eventually your friends got bored or irritated, but you wanted to keep playing. Likewise, I assume that more recently you've been doing some reading and now you recognize your youthful leanings for what they may have been: early indicators of later inclinations.
These types of fantasies are fairly likely to be lifelong, but like any other enthusiasm they are apt to wax and wane with the seasons, the hormones, and the circumstances. One of those circumstances may be deprivation, but I have to say that it's just as likely to be immersion if sex breeds sex (and it does), then kink no doubt breeds kink as well. Therefore, indulging in online simulacra or other noncorporeal outlets is not necessarily a cure for inappropriate fantasizing. (Hold that thought.)
"Wait," you say. "What's so inappropriate about S-M fantasies? I thought Andrea was kinda in favor of those?" Maybe I am and maybe I amn't, but that's beside the point. It is obvious, given your commitment to your marriage and your wife's lack of interest, your power-play longings are not doing you any favors, so dwelling on them may not turn out to be very helpful. Individual real-life appropriateness aside, I actually think S-M is morally neutral: great for some people, a bad choice for others, and, as my Hispanophone friend Melissa would say, bla bla y bla.
Now, is it really a bad idea to immerse oneself in S-M fantasies if one will not be indulging them in real life? No, of course it isn't. If there is one tenet by which all sex educators swear, it is that fantasy is fantasy and reality is reality, and there is no obligation that ever the twain should meet. If, however, the fantasy ignites and will not quiet, and you find yourself spending ever more of your precious waking hours obsessing on it, then cultivating a very rich fantasy life is probably not for you.
Ah, but you didn't really ask about fantasy. You asked about finding a real person, similarly unfulfilled at home, and embarking on a S-M-only clandestine nonromance. And I say, in the immortal words of Rocky the Flying Squirrel, "That trick never works!"
Is it possible to have a partner with whom one only does S-M, no sex, and with whom one does not fall in love? Emphatically yes. Is it a good idea to do this without one's spouse's agreement? Of course not. Add in the special intimacy, false or not, that you and such a partner would likely forge, based largely on the seductive call of "my partner doesn't understand me," and really, just no. I didn't miss the part about your worldview and faith being incompatible with acting on any of this, either. Happily you do see that putting yourself through that many uncomfortable and potentially unethical contortions at once can only lead to injury psychic and possibly otherwise. I think.
I do not believe that acting out a power differential with a fully informed and consenting partner is incompatible with an egalitarian or nonviolent worldview, but if you do, that's going to be a bad fit. As for not fitting in with your faith, well, I'm unaware of any organized religions except perhaps what a friend once referred to as "Episcopaganism" that expressly embrace kinky sex, but many insist only that you respect your body and your partner's, an idea that is open to hairsplitting interpretation. You would know best, of course. If what I'm hearing from you is what you meant to present, though, I'd have to say that a moderate amount of (porn-assisted, if you like) fantasy and no real-life contacts will be the healthier choice for you. Finding a girl on the Internet and flogging her? Not gonna help.
Andrea is home with the kids and going stir-crazy. Write her a letter! Ask her a question! Send her your tedious e-mail forwards! On second thought, don't do that. Just ask her a question.