I don't know if you'd consider Mexico, but from the way you describe yourselves I think you would really LOVE Tulum, Mexico. You fly into the Cancun Airport (3.5 hrs. direct from Dulles) - then taxi through Riviera Maya, through Playa del Carmen, then you hit Tulum.
It is truly one of the most special, natural, romantic places I have ever been. The beach is as good if not better than any you'll find in the Bahamas - looks straight out of Gilligans Island. Because of the proximity to the Mayan Ruins, the beach town is under UNESCO protection - so it's all eco-run. It's "eco- chic" all filtered ocean and rain water used etc. The food is fantastic. At night, all of the open air restaurants are lit only by the night of the moon and lots of lanterns. Lots of yoga and spas.
If you consider it for this trip or the future, two hotels to look into are: Ana y Jose
http://www.anayjose.com/ and Las Ranitas
http://www.lasranitashotel.com/
I'd also chime in that Atlantis is a bit Disney-like. Don't get me wrong, the sheer size and grandeur is pretty amazing - incredible architecture - but it's very commercial and crowded. From Paradise Island, you can take a bridge across to downtown Nassau. It has it's charms -- but again, it's very commercial and a huge cruise ship destination. I'm not sure if it's the relaxing, romantic destinaton you're searching for...