I'm having a difficult time putting this in terms I can explain clearly, but I somewhat think that the link that Ryan provided may have some inherent sampling bias. My thought revolves around the fact that the construct of marriage itself may have societal connotations in the sampled countries. Meaning, what it means to be married isn't equal across the sample.
Without question, there are biological dispositions driven both chemically and psychologically. Hell, pheromone research alone supports the utility of these "identities." I guess what I'm saying is the sexual organ is less important than the associated dispositions, but these dispositions tend to correlate naturally.