There are a number of average sex differences in personality traits that would contribute to more males identifying as "rationalists" than females.

Here are the sex differences found in the Big Five personality inventory, from a cross-cultural survey by Costa et al.:

• Women score higher on Agreeableness

• Men score higher on the Assertiveness facet of Extraversion

• Men score higher on Openness to Ideas, especially in the US. Women score higher on Openness to Feelings and Openness to Aesthetics. In the US, men also score higher on Openness to Fantasy.

• Some particular items, such as identification with the word "logic," were skewed strongly towards males

An interest in rationality may depend on Openness to Ideas. Otherwise, someone just isn't going to care about the kind of things we talk about here.

Furthermore, the identification of males, but not females, with words like "logic" suggests that perhaps part of the gender gap of interest in rationality is about words like "logic," and "rationality." Women are often labeled as "irrational" or "illogical" when they are perceived as overemotional, and this labeling may put them off words like "rationality," regardless of whether they appreciate the underlying thought processes of rationality.

Another major sex difference relates to Simon-Baron Cohen's theory of autism as an example of the "extreme male brain." Baron-Cohen argues that males tend to be higher in "systemizing" traits, while women tend to be higher in "empathizing" traits:

Empathizing is a drive to identify another person's emotions and thoughts and respond to them appropriately. Systemizing is a drive to analyze systems or construct systems. The Empathizing-Systemizing (E-S) model suggests that these are major dimensions in which individuals differ from each other, and women being superior in empathizing and men in systemizing. In this study, we examined new questionnaires, the Empathy Quotient (EQ) and the Systemizing Quotient (SQ). Participants were 1 250 students, 616 men and 634 women, from eight universities, who completed both the EQ and SQ. Results showed that women scored higher than men on the EQ, and the result was reversed on the SQ. Results also showed that humanities majors scored higher than sciences majors on the EQ, and again the result was reversed on the SQ. (cite)

Here is an interesting summary from Baron-Cohen:

Evidence is reviewed suggesting that, in the general population, empathizing and systemizing show strong sex differences. The function of systemizing is to predict lawful events, including lawful change, or patterns in data. Also reviewed is the evidence that individuals on the autistic spectrum have degrees of empathizing difficulties alongside hypersystemizing. The hypersystemizing theory of autism spectrum conditions (ASC) proposes that people with ASC have an unusually strong drive to systemize. This can explain their preference for systems that change in highly lawful or predictable ways; why they become disabled when faced with systems characterized by less lawful change; and their "need for sameness" or "resistance to change". If "truth" is defined as lawful patterns in data then, according to the hypersystemizing theory, people with ASC are strongly driven to discover the "truth".(cite)

This sounds like a rationalistic cognitive style.

If autistic-spectrum traits, or "systemizing," are related to interest in rationality, and in identifying as a rationalist, then it would be unsurprising that females are less likely to do those things.