Without making any value judgement on the merits of the legislation, many of those declaring a law as unenforcable seem a little unsure about how the criminal justice systems actually works. It's a deterrent; if you get caught, you get done. The law forbidding murder, or the use of mobile phones whilst driving, is largely unenforcable. The point is, if you get caught the book is thrown at you.
Enforcability does not require a copper with truncheon to witness the event. There are plenty of scenarios which, after the act, could lead to a punter or prossie getting caught. Infecting your OH could be the start. If the deterrent is strong enough, it is highly likely that the German legislation will result in a reduction in BB and a resultant reduction in STI infections. There will, of course, always be a risk, but minimising this is no bad thing in my book.