Rovinj is on of the most romantic towns on the Adriatic coast. Rovinj has a unique blend of urban and fishing settlement architecture, numerous bars and restaurants and we highly reccommend it as a romantic short or weekend getaway.
Rovinj

In the well-indented coastal area with a number of small bays, hidden within dense vegetation, you will often come across naturists. Although the beaches are not specified as naturist, many people choose to enjoy the sea and the sun in this way with a reserved toleration of other swimmers. 
Excellent Rovinj beache guide can be found here.
Rovinj beach

Rovinj beach
Romantic couples seeking an all naturist holiday, can stay in Valalta naturist resort (apartments, camping). Valalta is situated 8 km north of Rovinj's center.