On Maui I would recommend the Wailea area, but it's pricey. Kapalua is also nice, but windy. Ka'anapali, where the majority of the hotels are, is really touristy. It's fun, but I would not go there for a honeymoon.
The Four Seasons, the Kea Lani & Kapalua Resort on Maui are the most romantic. Kea Lani is an all suites hotel, which is nice. Kapalua also rents condos and houses with ocean views. I love the Grand Wailea too, but like the Hyatt, it's packed with kids because of their awesome pools.
We've also rented small houses on the beach on Oahu, Kauai and Hawaii. You usually don't get a pool, but we've stayed in a couple private beach front cottages on the Big Island that are great for honeymooners.
IMO, the most romantic resort in all the islands is the Four Seasons Hualalai on the Big Island. The rooms are in duplex/fourplex buildings scattered along the coast. The ground floor rooms have an indoor/outdoor shower (completely private). The resort is beyond beautiful. Love it there! If money is available, it's the perfect honeymoon destination.