I'd say it's all in context of the situation. One makes their own morality, and I'm not sure what your personnal situation is (if this in fact deals with you).
Let's say the husband is Mr X
the wife is Ms. Y
And the, um, third party is Ms. Z
I would say it is Mr X's duty to keep his wife Ms Y informed of what he is doing. It's not Ms. Z's concern whether he does this or not, unless Ms. Z and Ms. Y happen to be friends or something. In which case I would say Ms. Z should be careful about getting into a sticky situation.
Who knows? The wife could be very well aware of her husband's, er, side projects, and even approve of them. And swinger's clubs exist for a reason.
I can say this with a little experience because I have been a third party in a very dramatic triangle that I should have stayed the hell away from. I fell in love with someone who was already "taken." I have never agreed with the possessive aspects of relationships, but that's just me. I think as an individual you have to take all things/people into consideration to come to your own conclusions about what is "right" or "wrong" here.
"It's too bad stupidity isn't painful." -Anton Szandor LaVey
"We are all animals, my lady!" -Darkness, "Legend"