I totally agree with you on the one sided romance of the Serah and Hope parts. The thing is that they couldn't match together despite all the moves made by Serah to make it blow into a love relationship.
I agree that romance is often done in an infantile (or fairy tale book) way. I didn't mind it so much when I was younger, but it often makes me cringe now.
In any case, I don't care much for romance and am totally fine if the game lacks it.
There are lots of romance in Final Fantasy games, in fact I believe Square Enix tried to structure the game not to be all about fighting and so the inclusion of love and romance in it. Some of character in love which are very popular in the game are Cloud Strife and Aerith, Yuna and Tidus, Squall and Rinoa.
The only romantic relationship that I know so far in the Final Fantasy series is TidusXYuna and SquallXRinoa. It's a shame that Final Fantasy XV's romance is almost nonexistent because the story's focus is Noctis and Luna's wedding.