I would recommend Villa del Sol and La Casa que Canta in Zijuatenejo. Both discourage children and are on what I consider to be the best beach on the Pacific. They are both fairly pricey ($US300+ per night) but worth it. The hotels are small, intimate and great for couples. For even more seclusion at a higher price, both Las Alamandas north of Puerto Vallarta and Maroma, south of Cancun are both great.

For less expensive options, Villas Fa'sol and the Quinta Real in Huatulco provide a fair amount of seclusion, and Huatulco is small and pleasant, with the added attraction of some of the most beautiful beaches in Mexico, accessible only by boat.