See I think whatever floats your boat I can see why this would turn some on.
I can see why and am open to the idea as to why, I wouldn't though no way!
However my issue would be ok once in a blue moon as a "treat" sure.
But if my DH couldn't or wouldn't have sex for me for a very long time and the only time he could is living out this fantasy, I'd be mortified.
It says a lot about your realtionship tbh, and it's not good.
He has only come near you, for the first time in ages, because he can share the experince with others, cyber swinging/diggong style. Can't say I'd be filled with confidence about the future of my realtionship or self asteem if I were you.
He has only come near you when you agree to shag infront of whoever on the www, where is the intimacy in your relationship? Generally things like this, if one partner can or will only perform doing certain things it raises the bar, and becomes an issue.