the topic of conversation went there. In hind site it was dumb. She had been getting pretty graphic and telling me some outrageous fantasies of her own. She asked me for one and I came up with that.
you know what, don't apologize for the fantasy, you're being honest, it's just a fantasy, if she can fantasize, you are allowed to as well, view it as a test, especially in your situation where she is messing around with another man, that's some crazy double standard, think about it, she's allowed to be with another man but you aren't allowed to fantasize about a threesome - seriously, it's ok to be honest and fantasizing isn't a bad thing, don't get fooled into the trap of believing that your fantasy is wrong, there's nothing wrong it, seriously nothing wrong with that fantasy at all.
Let's be real about this, your wife is practically having a threesome, she has you and the other guy, that's 3 people to me.
Stick to your guns, there's nothing wrong with your fantasy, don't let anyone tell you different. She's not worried about your insecurities while being with another guy.