Hi Jess,
For me, the "concern" about whether the wife is on board with this (and obviously she appears to be) stems from the basic reality that fooling around with others, whether for money or not, is fundamentally risky and dangerous. I'm not talking about catching an STD but referring to the temptation of eventually (some might argue inevitably) finding someone else who's ultimately a better match, more fun, more interesting, more supportive of your passions, beliefs, etc. Let's face it -- if you're in a "committed relationship," you've mutually agreed to stop looking, stop "playing" with others and make the relationship work. You've vowed, presumably, to not allow yourself to be tempted elsewhere. And, let's be real -- despite how we want to convince ourselves, the person we happen to have "chosen" is certainly not the best, most compatible human being for us on the planet. There are millions of others who would tickle our fancy more -- be more interesting, more supportive -- if we were exposed to them. We simply came across the person we fell in love with and decided, 'you know what, I really like this, I think I'm really happy, and I'm staying put.'
Opening up to playing with others sexually, which ultimately is an undeniably intimate experience, is, in my view, playing with fire. How long is it before we come across someone else who is so fun and exciting, etc., that it gives us pause, makes us wonder... oh god, am I spending the rest of my life with the right person? Being in a committed relationship requires a certain amount of determined denial, of "I'm not going to go there with someone else." When you blow that off and jump into sex with others, you're risking your relationship. To what extent, I can't say. Maybe Nick and his wife have successfully steeled themselves to this activity and are at very little risk of threatening their marriage. I sincerely hope so. But a whole lot of people don't have that discipline.