Hi Luge. I hope you're still reading and really thinking.
So many posters hit the nail on the head. Please excuse the yelling but it's critical BEING A WITNESS ISN'T SOMETHING YOU DO; IT IS SOMETHING YOU ARE. IT PERMEATES 99.99% OF EVERY DECISION THAT IS MADE. This includes clothing styles, makeup, hair, car you drive, sex, own or rent home, vacations, reading, tv, movies, politics, news stories (OMG news stories, everything good or bad shows Armageddon is tomorrow), long term planning (there isn't much) and work (such as promotions, education, overtime, etc).
I would add that what happens a lot is that the witness who marries outside the Lord will begin to feel a sense of guilt after a certain amount of time (the amount of time depends on the person) and when they return to Jehovah they will be more involved and active in the congregation than ever before.
In her mind you are great guy and since you are a great guy she knows you will make a great witness. Everyone in the congregation and her family will work towards that result. You will be viewed as a potential witness for the duration of your relationship. I can't tell you how many unbelieving husbands I have befriended because we wanted to show them how normal witnesses are and how great it is to be one. I was really their friend, but it was conditional. Once they showed no interest or potential I moved on since I wasn't interested in having a worldly friend.
In her mind and in her family's mind you will be separating her from Jehovah and costing her everlasting life in a paradise earth. That's not a great foundation for a marriage.
Don't even get me started on having kids and raising them in that type of environment. Trust me, I do it every day!