Fornix also meant "brothel, bagnio, stew" in Latin. Fornicor means "to frequent whores, to fornicate". I've seen it suggested that brothels were in cheaper rent, underground, vaulted areas. There's a whole street full of brothels in Pompey and the graffiti on the walls is filthy (in Latin and Oscan).

This reminds me of one of my favorite terms for this activity: to swive; cognate with our modern word swivel. You see it used back in Early Modern English or Late Middle English.