It sounds like you guys are already engaging in activity that isn't pleasing to God. If she's worried about you staying pleased sexually, then you've probably already seen parts of her anatomy that you shouldn't have before marriage. Otherwise, you'd have nothing to compare it to and the reduction wouldn't be so mortifying for you. Fixing that issue should be your primary focus. On a practical level, if she has really large breasts (my wife is in the G's) then she will experience shoulder, neck and back pain that can be excruciating to the point of debilitating. She may also begin to experience migraines from the stresses on the musculoskeletal system. If she really wants this, I see no problem with it. Your "mutilating God's creation" argument rings hollow. It reeks of self-preservation, and thinking of yourself. Your focus should be on her. If you truly love her, you'll stop the premarital sex and support her surgery.