http://www.nytimes.com/2010/01/29/us/29sfmetro.html
Many *** couples negotiate open relationships - SFGate
These are just select examples but there's various statistics that you can look up that suggest open relationships to either be predominate over or equally as common as monogamous ones in homosexual couples.
I admit that I should have replaced "most" and supplement "many" twice in my last sentence.