I myself wouldn't think of it that way, but I can see how some people might. Regardless, I think the worst thing you could do is talk to the wife before talking to your friend.
Asking your friend if he minds you dating his wife is showing him that you respect your friendship above some hypothetical romance. It doesn't assume that the wife will have no say in the matter.
Asking a father to marry his daughter is showing him that you view the daughter as property. It assumes that she will have no say in the matter.
Marrying someone, in my opinion, implies that you have agreed to consider their feelings and respect their wishes, when reasonable. That means that if your husband thinks it would hurt his friendship if you date his friend, then you respect that... not as property but as a caring, respectful partner.
Having been in a similar situation recently, I'm speaking from personal experience when I say that your best friend and your husband negotiating relations before asking how you feel can be hurtful if not done carefully... and that's when they didn't even actually discuss it, just allowed the sexual tension to reach a point where he felt it had to be discussed with me. Then I felt pressure from myself to approve of the activities, on the basis that I'm poly and that's "the poly thing to do." Where my situation diverges from this situation is that I ultimately realized the source of my discomfort had nothing to do with it being my best friend and husband, but rather her current situation combined with my impression that they were behaving naively.
Since this couple sounds practised at these conversations, I agree that telling them at the same time but giving them a chance to discuss before they respond is a viable option. However, having personally been in the situation, I still think it's better to ask the husband first if he would mind. I just think of it the same as when you're in high school, and your friend's crush asks you out on a date. Your friend doesn't own you or her crush, but the respectful thing to do for the friendship is to ask your friend how they feel about you going on a date with their crush, knowing they might put the kabosh on it... and then respecting your friend's wishes.
Also, emphasize that their marriage and your friendship are more important to you than a romance with his wife, and that you will not make him feel bad or guilty if he isn't comfortable with it.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker
Last edited by SchrodingersCat; 06-26-2012 at 06:57 AM.