Luca is for several years in a row already the most popular name for male newborns in the German-speaking part of Switzerland, and people here have no trouble to recognize it as a male name, despite the "a" ending. See here:
http://www.bfs.admin.ch/bfs/portal/de/index/themen/bevoelkerung/uebersicht/blank/analysen__berichte/prenoms/01.html
I did not research it, but I have a nagging suspicion that the origin of the female Luka/Luca in Germany is not a proper, genuine variant of clearly female
Lucia and Lukretia, but simply the result of a (still on-going) confusion: If in a country enough people mistake a male name as female, it gets female there as well...
Rene www.AboutNames.ch