You know, before we married, I asked DH about porn use (because that would have been a deal-breaker for me) but didn't ask him about masturbation (because that honestly didn't seem to be "any of my business"--his body wasn't mine yet). I did tell him I wanted the right of first refusal after we got married (and I've never told him no, go take care of it yourself).
I'm not saying that all fiancees "should" feel that way, just that I did. But my personal opinion is that one cannot bind another's conscience with a "feeling" that something is sin. (I had a friend who tried to convince me that it would be wrong to hold my FH's hand; I considered it to be her private opinion and not binding on me.) If you have clear biblical evidence that something is wrong, then show it. If you know it's a debatable issue but it's one you cannot live with, that is relevant after marriage. (If you think drinking is biblically permissible, but you don't want wine in your own home, that's an acceptable position.)
My husband and I are in the age group where women's sex drive is as high as men's or higher, so I have no issue with fully satisfying him. If we were younger, we might have had to deal with "what will we do if I am pregnant / having my period / sick for two or three days, or if you want it three times a day and once or twice is enough for me?" In fact, if I was not inclined to be generous, he might have had to deal with "what if I want it three or four times a week and you think once every week or two is enough?"
Since this is an issue where you disagree, and an issue that will probably continue to be relevant after marriage, at least periodically, it is something you need to work out. If I were talking to your wife, I would say this: "Your husband will be the leader, the head of your home. You are going to have to trust the way he leads your home. Can you accept his leadership sexually and submit to his choices in that leadership? (That does NOT mean that if he decides he wants something that you find over the line, that you cannot say anything and you 'have to do it.' But it does mean you can relax into his leadership and know he has your best interests at heart.) Obviously if something is morally wrong, such as porn, you will be free to insist that he does not do it, and even to call in church leadership if he is involved in unrepentant sin. But in 'gray' areas such as masturbation, do you feel 'safe' enough to let him make the ultimate decision, or would it be a deal breaker for you if you knew today that a year into marriage your husband's drive would be a little higher than yours and he would be masturbating two or three times a month?"
Because honestly, unless you are convinced it is actually wrong (even if that it is simply wrong because it hurts her) and that is enough power for you to stop doing it, OR she is able to let you have freedom to make the decision, this will continue to be a conflict. If you don't do it in order to placate her (but not because you actually believe it is wrong), you could easily either resent her or do it anyway (either in secret or not caring whether it hurts her). Ultimately you need to lead in this, and she needs to "accept" and follow your lead. (I would counsel her from this angle.) If this would be a deal breaker for her, then this is something you need to work through.
Marriage--what a wonderful image of Christ's love for His bride!