Another factor that could be contributing to that statistic, is that there exists cultural pressure to be in a heterosexual relationship, and there also exists cultural pressure to not be in a homosexual relationship. I could easily imagine this skewing the sample even further, by creating a much higher average threshold of wanting to be together necessary for homosexuals to enter the category “couples”.
Addendum:
Another factor that could be contributing to that statistic, is that there exists cultural pressure to be in a heterosexual relationship, and there also exists cultural pressure to not be in a homosexual relationship. I could easily imagine this skewing the sample even further, by creating a much higher average threshold of wanting to be together necessary for homosexuals to enter the category “couples”.