I think it's pretty silly to order a 17 year old girl not to have sex, but this sort of thing happens all the time. I've represented hundreds of juveniles in these types of proceedings and this type of ruling doesn't surprise me a bit. The girl is a minor and she is under the jurisdiction of the court. She's probably gotten in a lot of trouble and the judge is trying to put her on the right path. Her parents may be worthless, or they may have actually asked for such a ruling. This is not "legislating" from the bench or making up laws. The judge has discretion to make such orders as necessary when he or she believes they are in the best interest of the child. Whether ordering a kid not to have sex would be found unconstitutional, I don't know. I doubt it, not under these circumstances.
When it comes to restrictions placed on an offender via probation, the restrictions must be related to the underlying offense and the desire of the court to prevent further offenses of the same type. Judge/courts simply do not have a blank check to place any/all kinds of conditions of probation on offenders simply because they are the judge & they say they can do it.
There's loads of case law stretching back several decades on this point, including on the specific issue of when judges can issue sentences that affect the ability of one to procreate.
Whether ordering a kid not to have sex would be found unconstitutional, I don't know. I doubt it, not under these circumstances.
Under what circumstances? Offender commits a drug crime so no sex? Does that sound like a rational nexus?