Questions:
Who is sub/dom?
How far into this does she want to go. There are levels of bdsm that extend from mild tying each other up to Marqués de Sade.
Did you know your wife had this kink before or after marriage?
If she wants to dildo your ass and/or physically hurt you, understandable that you are not into it and you have every right refuse.
If she never expressed it before now, then you should have no guilt about saying no and she should not have any resentment.
If she did express this kink before marriage or at the beginning of the relationship, (and it is not too far outside your boundaries) then the advice is you should be a good, giving and game husband and indulge your wife now and again. It is called compromise for the sake of making your partner happy and sexually invested in you. Not all the time, but you should be able to come to some sort of arrangement. Hey, you never know, you may grow to like it.
If she wants it to be a staple, then it borders selfishness and if some sort of compromise cannot be agreed to, therein lies you problem.
Her seeking outside fulfillment for her sexual proclivities is an entirely different matter and her suggesting and/or pushing for it is troublesome.
she already had someone in mind!
Whenever they bring it up first, you can count on it.
I can't say no because I'm afraid she'll do it behind my back or resent me.
Bullshit, you can say no, you have every right to say no, and every right to not feel guilty about saying no.
If she gets her kicks outside the marriage without your approval, then she is cheating and you have grounds for divorce.
I don't know what to do
If you cannot indulge her sexual tastes and you do not want her getting them outside the marriage, and she refuses to adhere to her marital vows, then you most likely outcome is divorce.
Stand firm on your position, do not agree to something you do not want, and if she cheats she's out.
EDIT I guess I should have read the comments.