Why don't you tell her that the statistics are that the majority of people who marry before they are 27 divorce. I certainly did. I married at 19, had five great years and nine miserable before I finally divorced. I'm not unusual, except in my stubborn decision to stay married when it was clear that I should have divorced. 27 seems to be the magic number. After that the likelihood you will stay married soars. Try to avoid the whole gaudy topic of cross dressing when you make this kind of decision. Cross dressing pales in comparison to immaturity. Even worse you have been together since you were 17! The number of people who married and stay married to their high school sweethearts is vanishingly small.
And your last statement that if you get married you will stay married really, really suggests to me you should wait. If you were content with a starter marriage maybe that would be different. Developmental changes between 22 and 27 are considerable. You might not at all be the same person at 27 as you are now. Marriage is easy. Divorce is hard. Give yourself 5 years. What's the difference? You are absolutely correct that she is likely to leave you if you get married, but not because you are gender variant, but because that's what happens to the majority of couples who marry at your age.
