This seems to be about an established couple secure in their relationship who have started exploring swinging or polyamory. The song's narrator seems to have some prior experience in this and is aroused by the idea of his partner sharing pleasure with others -- not in a cuckold sense, but rather being aroused by his lover's arousal regardless of who it's with (i.e. compersion). His lover is new to this and started out rather nervous and uncertain, but after a few encounters is starting to gain self confidence and revel in it, supported by his assurance that it is not a threat to their primary relationship.
This seems to be about an established couple secure in their relationship who have started exploring swinging or polyamory. The song's narrator seems to have some prior experience in this and is aroused by the idea of his partner sharing pleasure with others -- not in a cuckold sense, but rather being aroused by his lover's arousal regardless of who it's with (i.e. compersion). His lover is new to this and started out rather nervous and uncertain, but after a few encounters is starting to gain self confidence and revel in it, supported by his assurance that it is not a threat to their primary relationship.