Hi, if you're looking to get away from Singapore for a private couples beach retreat, you can look into Nikoi Island. It's a private island resort just about an 1hour ferry ride from Singapore. It's really an escape with only about 30 huts, no aircon, very rustic and the all-inclusive package means you can just sit back with your book and get massages every day.
I've personally been and it was a nice weekend respite away.