(I realize this is an exceptionally old thread, recently revisited)
It's exceptionally rare that I disagree with Couplers, but...
I would never, ever, ever, in a million years engage in swinging sex with a woman with a significant chance of getting her pregnant, or likewise a man to do so with my wife. Ok sure, your wife gets pregnant, she has the child, you raise the child as your own. Doesn't sound too problematic, does it? Except, it is. Consider;
The child won't be certain of half of their parental medical background. This could cause problems later in life.
The child might harbor lingering concerns their dad is not their dad, and the dad might not ever know their concerns.
The child might some day get a DNA test. What if the parents are no longer among the living? Questions arise that will never have answers.
Same concern; what if the parents are still alive and they ask? Now the child will know their father was a random fuck for their mom.
For me, this is just the beginning of the potential problems. I won't inflict that on an innocent child-to-be. My wife and I have always taken strong measures to ensure we don't get pregnant and don't get others pregnant.