Okay, so this one is for the ladies as well as the gents:
How did it make you feel to see you partner having an orgasm with another person?
For the Gents: seeing the pleasure on her face the moment another man penetrates her and the orgasmic pleasure later..
For the Ladies: Seeing the pleasure on the other Lady's face at penetration, as well as her juices on your man's cock. Also watching your man ejaculate at orgasm, knowing another woman brought him this pleasure