It probably depends on where you are in the situation and the people involved. For example, if it's still early in an affair and things haven't gone too far, or the spouse involved is kind of on the fence about things, putting down boundries may help. On the other hand, if the A has developed, the spouse is beginning to lean towards leaving the M, putting down boundries may just give them a convenient excuse to leave (of course, at that point they will look for any excuse).

Sometimes the best thing to do is look very carefully at motivations, individual risk-taking or non-risk taking tendencies, where things are with the relationships involved, and then try something and guage the reaction. Pay attention to what seems to work, and what doesn't work, and then react based on that.

There is no arriving, ever. It is all a continual becoming.