""They can't even stop people in jail from having sex - they're going to stop an amoral 17-year-old? It's not realistic.
Moreover, I don't think there is any statute authorizing a judge to impose "no sex" conditions. Furthermore, the courts themselves have clearly stated that legislatures cannot legislate in that manner (Lawrence v. Texas). ""
You are mixing too many things together. This has nothing to do with legislation, which is a process whereby laws are developed and put into place, and a judge does not need a statute to place restrictions on probation.
I agree you can't stop anyone from doing anything (well, locking a 17 yr old up can stop her from leaving) - does this mean we should abandon all laws, prisons, etc., and just forget about it? If the girl is so desperate she will have sex in prison (forcibly done against her, notwithstanding), then yes, what can be done. But to jump to the original issue to this proposition seems a stretch.
Can a judge make an order about something where Congress is prohibited from making a law on that subject?