Immediate reaction, NOPE, not okay.... Frankly, as a man, I can't begin to fathom why any man would want to "share" his wife in that way. "Share" his wife in any way for that matter...
My advice/opinions tend to be heavily influenced by the thought process...What would I do if I woke up in your/his shoes?
If it's something you've talked about as a couple or he has shared with you as a fantasy of his and you're open minded, or want to fulfill all of his fantasies, etc. then that's completely different.
Many men have this "fantasy" of sharing their partner with a third person.
I don't think there is anything that is "very wrong" for either of you to ask the other. Communicating openly and honestly with one another is probably the single most important aspect to any successful relationship. So I would continue to encourage it. You don't have to like what is being said, but listen to your partner and then express how you are feeling about what was just said. Be careful not to react or over react too quickly as that might cause your partner to shut down and not communicate with you in the future.
Me personally? I'm not "sharing" the woman I love with anybody and I have zero fantasy/thought about ever doing so.