Many of the Adult Resorts, with the exception of Desire, do allow Singles, although if traveling alone you will be charged a “singles premium”.
Some of the Swinger’s Group Tours are to resorts that will allow singles, and some are not. However, unless specified, the group tours are typically designed for couples. There are occasionally singles tours, and when they happen, they are prominently featured on the website.
One example of a Single Friendly Resort is Hedonism II.