Cuckold/compersion fantasies
Cuckoldry is when one partner (often the man in M/F couple) is turned on by their partner having sex with someone else, often with an element of humiliation (“she’s cheating and I’m powerless”). Compersion is sort of the positive flip side – being turned on by your partner’s pleasure with others out of genuine happiness (no humiliation, just excitement). Without actually involving others, you can feed this fantasy via dirty talk: the hotwife might describe how another person flirted with her today and how it made her excited, or the cuckold might be tied to a chair while the wife pretends to talk on phone with her lover, etc. If you do involve a third person actually, be extremely sure this is desired because it can stir real jealousy. Some couples do find it fulfilling: one watches while another man/woman pleasures their partner. The watcher might have rules (like they’re bound and can’t join, or they serve after the act cleaning up). It’s psychological fetish territory and needs trust and clarity. As always, a heavy dose of reassurance (during and after) is needed – e.g. the spouse emphasizing “You’re the one I love, this is just a fantasy scenario.”