Telling your spouse that you will not condone/accept her pursuit of infidelity isn't doing something just for "her own good." It is recognizing that some actions by your spouse will elicit a boundary response from you.
All you have to do is look around you to see that marriages aren't indestructible. To me, it is reasonable to think that each spouse must make efforts to make the relationship strong.
There are often unavoidable things that many marriages have to deal with that have a negative impact: illness, financial distress, deployment, work associated travel, extended family issues, work related demands, etc.
Then there are the things we choose, often blind to their impact, that push the marriage into critical mass. Money/time consuming hobbies, barhopping with the guys/gals, friendships with the opposite sex, sexually charged flirtations, swinging, recreational choices that exclude the spouse, etc.
Pushing/encouraging your spouse in flirtations with other people is the equivalent (IMO) of taking a wrecking ball and bouncing it against the side of your marital "house."
At some point, the wrecking ball is going to breach the walls and your marriage will suffer the consequences.
If you (and I'm speaking hypothetically in all this) and your spouse expends time and energy into outside pursuits *without* a reciprocal investment of time/energy into the marriage, the marriage will eventually suffer.
