Crza96 - I hear what you're saying man!!!
Neither am I into being submissive or cuckolded but am very much into giving my girl the best experience possible - my concern is will I get jealous if/when we go to actually cross that bridge of a real MFM.
Likewise, after hearing my girls fantasy, I bought a number of large dildo's, including 2 that I thought would be so big that she wouldn't use them. The 2nd biggest one is black - I thought she might like that - after using it the first time, she made a comment about how good a BBC feels. I think that's part of the myth perpetuating the myth, the biggest one is a 'white' cock. All of them felt good to her, her preference isn't for the biggest one, but when she does want a real big one the BBC is it, but really because it's a little more manageable. Her favourite is me, then there is the next size up which is about 6"+ of girth that makes her go 'Oh my gosh' when it brings her to orgasm, but sometimes she really doesn't want the bigger one. And as I said, most of the time she just wants me, not only because it's real but because of the emotional connection.
So, I wonder about the dynamics of bringing in another M, especially if he was bigger - which is really the main purpose, bigger being better and a worthwhile experience. However, after reading through lots of posts, although bigger can be better, it's really only part of it being better. Connection, respect, love, a BBC can't replace that - I would be careful to make sure that my girl doesn't build too much of a bond to the other M - regardless of how big he is.
A guy I know is 9.75" long and quite thick, his wife left him and the guy she left him for had a small penis. A couple of other mates have large ones and their girlfriends have left them too. All I mean by that is that for most women, a BBC or BWC isn't the be all and end all, although, I'm sure most of them would enjoy the experience, hence your, my and the OP's posts here
Finally, my girl enjoys a big dildo, but some of her most intense O's have been with a finger or my cock. Her first O with the bigger dildo was also intense, but not every O with the bigger dildo is that intense. So every girl is different, some would take quite a while to adjust and may find it too much, my girl seems to be able to cope reasonably well with a very large one, but she still prefers mine overall. From what I gather from readings here and other observations, I think most wives prefer their partners regardless of their 'size', with some exceptions of course.
That being said, I very much enjoy being able to provide the ability for my girl to experience a really really big one. Oh, and in our case it was her fantasy which she told me with much encouragement from me to share what her fantasies were - of which a really big cock was only in a few of them - most of the fantasies were other dynamics.
Sorry it's so lengthy but hopefully it's of some use.