File this under a phenomenon desperately seeking an explanation.

According to data collected by the online dating service OKCupid, people who carry Apple (AAPL) iPhones have more sex partners by age 30 than owners of Research in Motion (RIMM) BlackBerries or Google (GOOG) Android phones.

Women with iPhones had the most partners of all — an average of 12.3 by age 30, compared with 10 partners for iPhone-wielding men. Men with Android phones have the fewest sex partners — 6 by age 30.

The findings come from an OKTrends report issued Tuesday whose main focus is what kinds of photographs are most likely to get you a date (answer: shallow depth of field shot without flash in late afternoon with a digital SLR). The sex-partner portion is presumably based on answers given by OKCupid users when they sign up.

The dating service has 3.5 million active members, but only 9,785 used smartphones to shoot their profiles. According to OKTrend’s Christian Rudder, that’s enough to show that the smartphone-brand-sex-partner pattern is not just a matter of older, more experienced people choosing one phone over another. Rudder explains:

“We found this data as part of our general camera-efficacy analysis: we crossed all kinds of user behaviors with the camera models and found we had data on the number of sexual partners for 9,785 people with smart phones. We dropped what we found into Excel, and voila.”

Below: the plot by age.

Having more sex partners, however, does not mean you are having more sex. Quite the contrary. Studies considerably more scientific than this one have consistently shown that married couples have more sex — and better sex — than singles. See for example here and here.

[Follow Philip Elmer-DeWitt on Twitter @philiped]