I'm quite saddened by the number of people on this thread who talk about the fun of being their lover's secret lover. Also by the justification that it's an okay thing to do because even if the affair is found out, the pain caused to the unknowing partner of the lover will probably be a growth experience for them.
Bloody hell - have some compassion. People report being lied to by their partner about a fundamental part of their life together as utter devastating. It isn't something that they tend to get over quickly and in some cases, they don't get over it at all. I can't imagine why anybody would want to be involved in bringing that level of hurt into the life of a complete stranger who has done you no harm.
I get that you guys want to have additional relationships but I very much don't like the attempts to justify doing that in a way that is likely to introduce a whole pile of hurt into somebody else's life.
Yuck, yuck, yuck.