Never have I offered to fuck someone else's wife and never would I do so.
First of all, if, in theory, I was to offer to fuck a friend's wife, I can almost guarantee that would be the end of our friendship and I would forever be seen as being a arrogant arsehole.(and for good reasons)
Having said that, I have been approached by people on-line about this and in 100% of the cases, it's been by the male (husband or boyfriend), never by the female in the relationship. This is a fantasy some men have, most likely stemming from their own fascination with a bigger dick and their belief that a little difference in penis size must make all the difference for the female on the receiving end.
Fantasies are good and fun but if they were to be acted on, in real life, would probably cause a lot more harm than good. Besides, in my own opinion, it's also very arrogant to think that just because you happen to have a larger dick than most, you're automatically superior in love making