Even assuming all of the participants are being truthful, you'll have some problems with definitions not being shared by the participants. The classic example is "Does oral sex count?" Men, for whom a large number of partners is (sometimes) culturally good, may be inclined to count it. Women, for whom a large number of partners is (sometimes) culturally bad, might not count it. The only halfway plausible explanation is that the 3.4% of men who are gay or bi all sext with each other.
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