The main time for swingers is traditionally August and the two weeks before, because that was when the French took their holidays.
The introduction of the compulsory 35 hour week in France meant the French took longer holidays, which lengthened the high swinging season at Cap to cover the whole of July, August and into mid-September. The fallout from the recession reduced the high season to just August. It’s always likely to fluctuate depending on the economy and the weather in any particular year. But if you’re going for the swinging we would always recommend the second half of July and August.
As it’s the south of France, the sunshine is reliable but this is still Europe and they can be rain and overcast days. Some years the sea is warm but in others a little more refreshing! The average temperature is above 25° in July and August with daily highs over 30°, but do check the forecasts through your favourite weather app before booking your own trip.