If it's privacy you're after, check out a few of the options on Maikhao beach (none of them are luxury though). The beach itself is a little coarse, but if you're going around the right time the water is good and it's a massively long stretch of beach that is hardly frequented. There are also very few resorts up that way, so most likely you'll have the beach to yourself and maybe a couple of other families.
Of course, there's always the FT-favs...Amanpuri (Pansea beach which is also barely frequented) and Trisara.