Completely anecdotal, but everyone I've known of that got into this sort of situation, be it cucking/swinging/voyeurism/threesomes etc:
A) Started because it was the man's idea and he got obsessed with it and wouldn't relent until she agreed to experiment.
B) Ended badly due to all kinds of unexpected consequences and emotions that couldn't be overcome.
Now usually the typical thing that happens is one person ends up developing an emotional and deepening relationship to an extracurricular partner to the point that they either leave the marriage for that person or the other partner can't handle it anymore. That's one big wild card here, you can claim you know this side kid really well all you want, but you acutally have no idea how he's going to approach this whole thing in the long run or how that might affect the marriage. Nor can you fully predict how your emotions towards both of them might change as this whole thing progresses. I know as a woman your emotions are generally going to be stable as a rock...but its a possibility!
I'm not really judging or telling you what to do, but its just my gut reaction when I hear about this sort of thing that "this won't end well". Wasn't lindz like a virgin before her marriage or something too? Maybe I've got that mixed up.
I also realize there are alternative lifestyle people out there who balance these relationships and fetishes perfectly well long term, but I get the impression they enter into that agreement from the beginning. Not after years of doing a more traditional and monogamous approach and then trying to branch into it.
Partially its just that I'm just a cynical and jaded personality, but I'd predict that the marriage is a shambles within the next two years the more this situation continues and evolves. Maybe not though, good luck!