You'll get varying -- and DEEPLY HELD -- opinions on this, P, but my own decision was that I was only ok with it if my wife and I agreed to it mutually, once we separated.
The funny thing, of course, is that whereas SHE was always the one that had wanted to (even said it was a "dealbreaker" if I would only do S instead of D if we didn't date other people), now all of a sudden SHE was the one that didn't want us to date other people!!!
