don't stress too much about some of the replies in this thread. if it's something you have a fantasy about and you thoroughly discuss the "ramifications" with your wife before, and she is willing, make sure you two keep the communication open after the playnite is over. many people here have failed at marriage while doing everything they are "supposed" to do. the one thing that makes any marriage work is communication. if you two can openly speak your feelings without fear of any type of retaliation, you two could do or go thru anything and more than likely come out better before you went in
there is nothing wrong with these type of sexual fantasies, partners wanting to see the other one with somebody else. don't feel like you're an oddball, there are a million people in this country who fantasize and act out on very similar fantasies like what you just described every weekend. i say if that's what floats your boat, go for it man