The motive for placing St Agnes of Rome into a brothel was not some unspecified "Roman law," but her stubborn insistence on being Jesus' bride. The officials wanted to threaten her and break her will. It was decided to execute her only AFTER she was put into a brothel, but not before. The story, by Prudentius, is here
http://www.ewtn.com/library/MARY/STAGNES.htm
The same goes for St Agatha
http://www.catholic.org/saints/saint.php?saint_id=14
In both cases, the goal was forceful conversion to paganism rather than defloration prior to execution. I believe that Roman law referred to the vestals only. They were buried alive for their offences, poor souls.

OK, I've checked it up. Seutonius indeed mentions this custom in Nero LXI (http://www.gutenberg.org/files/6400/6400-h/6400-h.htm): "according to an ancient custom, it was not lawful to strangle virgins, the young girls were first deflowered by the executioner, and afterwards strangled." So it was not only for the vestals. But it was the executioner's duty. Nobody put these girls in brothels to humiliate them. Suetonius also mentiones that those desirous to die, of shame, were let to live, as punishment.

Большое спасибо! Значит законы Холл приплел зря.

Спасибо.