Tell her it's not even an option you'll consider. You're firm on the answer being "no". You will not allow yourself to have your genitals mutilated, nor would you even allow it for future children.

I (an American woman) am in shock for you. Does she know anything about why circumcision is such a big deal? Does she know it's roots in sexual repression, and how those ideas are pushed on to infants!? Does she agree that little girls should be circumcised (we call the practice female genital mutilation, so just think on that)?

I didn't want to circumcise my son, which was fine with my husband, but my family was adamant I consider it. So, I did research.

I'm now even more adamant against circumcision (although I don't pull some justice warrior on it, unless it comes up as a pro/con debate) one because of the roots behind why, and two, because as a parent, how in the world could one of my first acts be allowing a doctor to do that.

Note - if you've never looked up how circumcision is done on infant boys, do. I'll try to find the medical journal I initially looked up. It made me vomit, literally. I couldn't stomach putting my son through that as one of my first acts as a mother. Needless to say, my son isn't circumcised.

On to the point though, what she's asking is akin to you saying she needs to get a boob job, or labiaplasty!, before you'll marry her. She's telling you your body isn't good enough. She's saying that she wants you to undergo a procedure that is very painful, that removes 10s of thousands of nerve endings, and that ultimately makes you feel unnatural.

Are you all waiting until marriage to have sex? (I'm guessing not from some context clues). Has she not enjoyed being with you? Was she shocked the first time she saw your penis and balked at intimacy because of it? I just don't get why, after three years, this is coming up now. Why is your penis fine as a boyfriend, but isn't as a husband?

I feel so sorry for you. I can't imagine being in your position.

If she's adamant about you doing this and since its a good idea anyway before marriage, take some premarital counseling with a sex positive therapist to really talk it over. Discuss why it's so important to her, discuss future children, all that. I don't jump on the therapy bandwagon often, but I think this is a case for it, if she won't hear your side in calm conversation at home.