I often see these "fantasies" posted by guys and I always think - "OK, thats your fantasy but how does your wife feel about it ?". Personally, I don't understand the motivation for the guy - I am definitely missing something here !!
I read a article in a newspaper about a threesome - this was "initially" fully consensual by the man & wife and with a friend. During the act itself the wife became very uncomfortable when the other girl was intimate with her husband. After the event there was a break in trust between the man & wife.
I think you are right to tread very carefully with this. There is a massive gap between fantasy & reality. This act could have long term impact on your relationship.
Read up on real stories not just Internet fantasy.