That was intentional, but I no longer think that's a good idea. The update I published today has made it possible to use the Romance Rose on the final contestant.
The original intent was to prevent you from skipping getting to know the contestant to date by simply popping everyone else and forcing them to date you, but it seems like nobody actually plays that way. The way you went about it was actually really sweet, and I'm sorry it didn't work out for you when you tried it!