Perhaps the frustrated husband who wants to experiment with bondage (having his wife do the deed) is only wanting to give away manly responsibility and control for a bit? In my mind this is almost like chastity devices for them, giving another control over something very personal. I can tell you that they get very compliant and submissive when locked up for a bit, but this isn't exactly manly for me either. It's fun and we've played around like this ourselves, but it's also nice when guys are guys; open the car door, take out the garbage, and walk on the street side of the sidewalk and hold my hand when we walk together...
Trading a husband for a maid in the short term can also be fun, but this to me is again self-demoting, taking a lesser (feminine) position as a vacation from manly responsibility. I can and do play along like this despite the obvious connotation that we're the lesser of the two genders, but not forever, and as you pointed out there has to be some give and take here, some perceived reward. I do get to flex my leadership muscles during these times, but that's also a burden. It's truly a chore to lead sometimes, which of course makes me sympathetic.
In the story the curious husband apparently already submits to his wife, but maybe in his mind submitting to another man is a further submission, a taboo submission. I could easily make the argument that this means the husband thinks men, (other men) are superior, not only to women, but to himself as well. To continue down this road (story wise) he has to know that he's trading something known for something unknown. Or maybe he should know this, but his lust for the taboo and unknown blinds him to the reality that is right in front of him.
Men are sexual creatures, they'll do almost anything for it, and with the right playful attitude and mood that can be a blast. As far as the threesome thing, I will humbly suggest that many men would like a second woman to prove their virility, the "I'm so much that I can handle you, and your friend" bravado. Now, if the guy in question maybe falls a little short on all that bluster, that maybe leads to a second thing that most guy just seem to love, the concept of two girls going at it with each other while they watch; if the girls are left needy. So, maybe in this particular case it's a logical fantasy for them, a win-win no matter how it turns out; again, from a certain point of view.
Other men maybe have a less grand opinion of themselves, they maybe want to share, maybe feel deep down that they could use a little help in properly taking care of somebody they care about, at least one time just to see what that looks like. I'm fairly certain that our teenage camping adventure was supposed to be one of those, but fate and a few misconceptions kind of derailed that crazy train. I'm not complaining as it was a grand adventure while it lasted, but obviously not for everybody.
So, here we are, two very reasonable adults that see this in a slightly different light, neither is right or wrong, I just think it depends on where exactly you stand in the proverbial picture.
Thank you again for your thought provoking words, Jackie.