Chaa Creek is terrific, although I would note that most of the units are in duplex cabanas which do not offer total privacy. The garden suite/honeymoon suite is private if you get both units, but it's pricey.
You might also look at Ek'Tun Lodge, which has just two private cabanas and the best swimming pool in Belize -- a natural pool hollowed out of limestone. You can often skinny-dip there with just the howler monkeys in the background for company.
Another romantic option in Cayo is Blancaneaux, in particular the villas. Gorgeous! You can even stay at Francis Ford Coppola's villa, if he's not there at the time.
Cayo Espanto is basically a part of Ambergris Caye, as it is just off the back side of the island, but once there people are reluctant to leave as everything is available there and included in the price.
Victoria House would be my choice for a romantic honeymoon spot on Ambergris -- especially the villa suites.
On the mainland coast, Robert's Grove, especially the deluxe condo suites, is terrific. Hamanasi in Hopkins is, too, and it's very similar to Robert's Grove as both were built by the same builder.
Belize books and publications by Lan Sluder:
Belize First Magazine (http://www.belizefirst.com
Fodor's Belize & Guatemala Guide (new 4th edition coming this summer)
Adapter Kit: Belize (the first and only comprehensive guide to living, retiring, working and investing in Belize -- a best-seller on Amazon.com)
San Pedro Cool (the first and only visitor guide to Ambergris Caye)
Belize First Guide to Mainland Belize
UpClose Central America
Belize Book of Lists