Went a-Googling and found http://socialpathology.blogspot.com/2010/09/infidelity-part-1.html (discusses findings of a cited study, not just blog ramblings I promise)
http://facweb.northseattle.edu/ratkins/ClassMaterials/PSYSOC230/230Activities-Instructor/13-Adult%20Years/a-Sexual_Infidelity_Married_Americans.pdf (see section on Opportunities)
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http://socialpathology.blogspot.com/2010/09/infidelity-part-1.html (discusses findings of a cited study, not just blog ramblings I promise)
http://facweb.northseattle.edu/ratkins/ClassMaterials/PSYSOC230/230Activities-Instructor/13-Adult%20Years/a-Sexual_Infidelity_Married_Americans.pdf (see section on Opportunities)
http://www.kinseyinstitute.org/resources/FAQ.html#Infidelity