I believe that in every case of pussy-sharing, there is an element of interest on the part of the husband in the "other" guy which is more than just curiosity. The very fact that he would provide his wife's pussy for the other guy to fuck indicates that he wants to "draw the other fellow in" WITH himself, in her pussy!
It is like they are fucking her together, and that their cocks are in contact and working together. And, if the situation evolves into a session in which the wife wants her husband to suck the other guy's cock, it seems most natural that he would do it.
So, what is wrong with a husband sucking another man's cock, even if his wife's pussy isn't present? Those men who do so have reported that while they suck the other guy's cock, they are still fantasizing about this "other" cock sliding in and out of their wives' pussies, and that their mouths are just substitutes for the missing marital pussy!
So, which came first: A husbands need to share his wife's pussy with other men, or his need to somehow "experience" those cocks in a more intimate way?