Researchers estimate that as many as 5 percent of Americans are currently in relationships involving consensual nonmonogamy — that is, permission to go outside the couple looking for love or sex.
The boundaries in these relationships are remarkably varied, with some couples negotiating one-off "swinging" or partner-swapping experiences. and others forming stable bonds among three, four or five partners simultaneously. The latter is a version of polyamory, relationships in which people have multiple partnerships at once with the full knowledge of all involved.
" Several studies show that couples in open marriages tend to be happier and more satisfied with their marriages and with life in general than couples in monogamous marriages."
from here: http://moderndirections.com/49.htm
"The benefits of swinging include increased marital satisfaction."
Read more: http://www.momlogic.com/2010/04/swinging_married_couples_a_good_thing.php#ixzz3692nCeKK
On the other hand, "While some couples are able to pull this lifestyle off, it’s a high risk proposition. Books like Easton and Hardy’s The Ethical Slut help couples to establish rules for open relationships; but the ability to follow them indefinitely is the key." see http://www.psychologytoday.com/blog/magnetic-partners/201307/the-slippery-slope-open-marriage