Yes, it certainly is.
Legalization of prostitution. Is that principled?
Very much so.
Legalization of sex with minors. Is that principled?
Mischaracterizing the libertarian position on this issue (as you have done here) is certainly not principled. The libertarian position is that if a minor can demonstrate that he can make his way in the world on his own, free of any parental dependence or guardianship, then that individual is qualified to determine whether of not he can engage in sexual intercourse.
Most parents state this exact position to their children when they tell them: "As long as you live in my house, you will obey my rules. When you get out and live on your own, you will be free to do what you want in those areas."
So basically, IMO, you are saying that when your daughter leaves the house at 18(The Libertarian Party platform actually states that any minor can declare their "independence" at any age) and she becomes a prostitute, you will proudly proclaim on FR, that your duaghter is a prostitute.