Based on the number of "attached" people I've seen on dating sites, I'd say it happens a lot. Although..that's probably more on the down low, not so much where everyone knows. But going back to my previous life, there were a lot of couples who were in "open" relationships, where they all knew everyone and one or both had other lovers on the side - openly.
yea, i always tell guys never marry til after 30, and vasectomy are reversible