Susan here: There are many social and psychological dynamics. For the man, he's reclaiming his woman and reconnecting. For the woman, it's a reconnection and communication that sex with another man has not effected her relationship. Then, there's just the fun of watching sex, getting laid, etc. There's been studies confirming that observing something is almost as good in the mind as actually doing it. This would explain the thrill of everything from porn to sporting events to movies. So, for a man to watch his wife have sex, then have sex with her himself, it's a total 'double dipping'' rush. Frankly, you can analyze this stuff to death, but the only quesitons are is it a positive experience for you and are you having fun. For me, there's something so powerful in that first moment when another man enters me with his wife/gf watching: permission, lust, acceptance and more distilled into a single moment of sexual intercourse. But hey, that's me and I gotta be me