the question was about realistic expectation, not how many they'd like to have

Recall that in the planning fallacy, there was no difference between the "realistic" forecast and the "best case" forecast. This form of self-deception extends beyond the area of sexuality; what question 2 actually measures is a person's vision of their next five years of sexual relationships, and how many and how enduring they'd want them to be. (The "realistic" disclaimer is commented out in conscious human forecasting.)

And, given the cultural incentives and evolutionary psychology, why would it be surprising for males' imagined "best case" to skew much higher than their actual future, and females' imagined "best case" to skew much lower?