I guess my biggest question here are all spouses aware of this? Those that are trying to hook up on this affair website? If not, then no I definately think it is wrong. To me, it makes no difference how you start an affair -- especially when you are hiding it.
Now if the other spouse knows about it and doesn't care, I am on the fence. Not whether it is right or wrong-- I can't decide someoné else's morals, but I have a hard time believing that for most people, this still wouldn't hurt a marriage.
Their motto is, Life is short, have an affair - pretty much sucks. You could then say Life is short, kick a puppy, Life is short, do drugs-- you could pretty much replace it with whatever vice or illegal or immoral thing you want to replace it with.