Cue hundreds of years of art history.
But don't you think that the more exciting it gets, the more those things become normal? As you quite rightly said she will probably do it anyway.
I know there are girls out there having sex at 14, but if you can prevent it, I would try my damdest.
But then i guess as you say ultimately you want to be supportive.
I would say the same as everyone else, make sure she has all the facts, and point out that her bf could be arrested.
For practical reasons, women began sporting pants for the first time soon after, drawing attention to legs like never before and making mainstream women's fashion at once more masculine and more erotic.