This would never happen to me, plus we are married, and it's not open. If my wife proposed it, I would be utterly stunned and uncomfortable due to the fact that she has never even hinted anything like that.
With that said, it is probably my dirtiest fantasy. When I do watch porn alone, these scenes are common. I am most turned on by scenes focused on the woman's (real or acting) desire for it, either from giving both partners blowjobs or DP. That is the only way I can imagine it, not with the woman being pressured or nervous in any way. Still, strictly fantasy for me, so the closest will be me, my wife, and a nice silicone dildo.