Sentence:  Some polyamorous people experience <WSD>"compersion</WSD>"<!--italics in original-->, which means feeling joy that one's partner is sharing closeness with another person (Keener, 2004; Polyamory Society, 1997). Keener's (2004) study participants noted an absence of worry as to whether their partners might be cheating ... 
Meaning of the word marked with <WSD> tags:  The feeling of vicarious joy associated with seeing one's sexual or romantic partner having another sexual or romantic relation that brings them joy, coined as antonym of (sexual or romantic) jealousy.