No, sexual activity is never compulsory at any Swing in Thailand after party.
However after an initial adjustment period you will be expected to remove your clothing down to your lingerie for woman or boxer briefs for men. And if in time you are not inclined towards any sexual activities with either your partner or any other guests it is polite to retreat to your own private villa.
It is typically frowned upon to remain clothed for lengthy periods of time and continue to watch other guests, unless asked!